diff --git a/Development/Shake.hs b/Development/Shake.hs
--- a/Development/Shake.hs
+++ b/Development/Shake.hs
@@ -9,7 +9,7 @@
 --main = 'shake' 'shakeOptions' $ do
 --    'want' [\"result.tar\"]
 --    \"*.tar\" '*>' \\out -> do
---        contents <- 'readFileLines' $ replaceExtension out \"txt\"
+--        contents <- 'readFileLines' $ 'Development.Shake.FilePath.replaceExtension' out \"txt\"
 --        'need' contents
 --        'system'' \"tar\" $ [\"-cf\",out] ++ contents
 -- @
@@ -76,7 +76,9 @@
     doesFileExist, getDirectoryContents, getDirectoryFiles, getDirectoryDirs,
     -- * Additional rules
     addOracle, askOracle,
-    alwaysRerun
+    alwaysRerun,
+    -- * Finite resources
+    Resource, newResource, withResource
     ) where
 
 -- I would love to use module export in the above export list, but alas Haddock
diff --git a/Development/Shake/Core.hs b/Development/Shake/Core.hs
--- a/Development/Shake/Core.hs
+++ b/Development/Shake/Core.hs
@@ -5,7 +5,8 @@
     ShakeOptions(..), shakeOptions, run,
     Rule(..), Rules, defaultRule, rule, action,
     Action, apply, apply1, traced,
-    Verbosity(..), getVerbosity, putLoud, putNormal, putQuiet
+    Verbosity(..), getVerbosity, putLoud, putNormal, putQuiet,
+    Resource, newResource, withResource
     ) where
 
 import Prelude hiding (catch)
@@ -22,7 +23,6 @@
 import qualified Data.HashMap.Strict as Map
 import Data.Maybe
 import Data.Monoid
-import Data.Typeable
 
 import Development.Shake.Pool
 import Development.Shake.Database
@@ -36,7 +36,7 @@
 -- | Options to control the execution of Shake, usually specified by overriding fields in
 --   'shakeOptions':
 --
---   @'shakeOptions'{'shakeThreads'=4, 'shakeDump'=True}@
+--   @ 'shakeOptions'{'shakeThreads'=4, 'shakeDump'=True}@
 data ShakeOptions = ShakeOptions
     {shakeFiles :: FilePath -- ^ Where shall I store the database and journal files (defaults to @.shake@).
     ,shakeThreads :: Int -- ^ What is the maximum number of rules I should run in parallel (defaults to @1@).
@@ -71,7 +71,7 @@
         [show inner]
 
 
--- | The verbosity data type, specified in 'shakeVerbosity'.
+-- | The verbosity data type, used by 'shakeVerbosity'.
 data Verbosity
     = Silent -- ^ Don't print any messages.
     | Quiet  -- ^ Only print essential messages (typically errors).
@@ -202,6 +202,7 @@
     ,execute :: Key -> Action Value
     ,output :: String -> IO ()
     ,verbosity :: Verbosity
+    ,logger :: String -> IO ()
     -- stack variables
     ,stack :: [Key] -- in reverse
     -- local variables
@@ -243,7 +244,7 @@
     let execute = createExecute rs
     withDatabase logger shakeFiles shakeVersion $ \database -> do
         runPool shakeThreads $ \pool -> do
-            let s0 = S database pool start stored execute output shakeVerbosity [] [] 0 []
+            let s0 = S database pool start stored execute output shakeVerbosity logger [] [] 0 []
             mapM_ (addPool pool . staunch . wrapStack [] . runAction s0) (actions rs)
         when shakeLint $ do
             checkValid database stored
@@ -342,7 +343,7 @@
 
 
 -- | Write an action to the trace list, along with the start/end time of running the IO action.
---   The 'system'' command automatically calls 'traced'. The trace list is used for profile reports
+--   The 'Develoment.Shake.system'' command automatically calls 'traced'. The trace list is used for profile reports
 --   (see 'shakeDump').
 traced :: String -> IO a -> Action a
 traced msg act = Action $ do
@@ -361,7 +362,7 @@
         liftIO $ output s msg
 
 
--- | Write a message to the output when the verbosity is appropriate.
+-- | Write a message to the output when the verbosity ('shakeVerbosity') is appropriate.
 --   The output will not be interleaved with any other Shake messages
 --   (other than those generated by system commands).
 putLoud, putNormal, putQuiet :: String -> Action ()
@@ -376,3 +377,22 @@
 --   not interleaved.
 getVerbosity :: Action Verbosity
 getVerbosity = Action $ gets verbosity
+
+
+-- | Run an action which uses part of a finite resource. For an example see 'Resource'.
+withResource :: Resource -> Int -> Action a -> Action a
+withResource r i act = Action $ do
+    s <- get
+    (res,s) <- liftIO $ bracket_
+        (do res <- acquireResource r i
+            case res of
+                Nothing -> logger s $ show r ++ " acquired " ++ show i ++ " with no wait"
+                Just wait -> do
+                    logger s $ show r ++ " waiting to acquire " ++ show i
+                    blockPool (pool s) wait
+                    logger s $ show r ++ " acquired " ++ show i ++ " after waiting")
+        (do releaseResource r i
+            logger s $ show r ++ " released " ++ show i)
+        (runAction s act)
+    put s
+    return res
diff --git a/Development/Shake/File.hs b/Development/Shake/File.hs
--- a/Development/Shake/File.hs
+++ b/Development/Shake/File.hs
@@ -92,13 +92,13 @@
 
 
 -- | Require that the following files are built before continuing. Particularly
---   necessary when calling 'system''. As an example:
+--   necessary when calling 'Development.Shake.system''. As an example:
 --
 -- @
---   \"//*.rot13\" '*>' \out -> do
---       let src = dropExtension out
---       'need' [src]
---       'system'' [\"rot13\",src,\"-o\",out]
+-- \"//*.rot13\" '*>' \\out -> do
+--     let src = 'Development.Shake.FilePath.dropExtension' out
+--     'need' [src]
+--     'Development.Shake.system'' [\"rot13\",src,\"-o\",out]
 -- @
 need :: [FilePath] -> Action ()
 need xs = (apply $ map File xs :: Action [FileTime]) >> return ()
@@ -106,9 +106,9 @@
 -- | Require that the following are built by the rules, used to specify the target.
 --
 -- @
---   main = 'shake' 'shakeOptions' $ do
---      'want' [\"Main.exe\"]
---      ...
+-- main = 'Development.Shake.shake' 'shakeOptions' $ do
+--    'want' [\"Main.exe\"]
+--    ...
 -- @
 --
 --   This program will build @Main.exe@, given sufficient rules.
@@ -121,9 +121,9 @@
 --   additional power. For any file used by the build system, only one rule should return 'True'.
 --
 -- @
---   (all isUpper . takeBaseName) '*>' \out -> do
---       let src = replaceBaseName out $ map toLower $ takeBaseName out
---       'writeFile'' . map toUpper =<< 'readFile'' src
+-- (all isUpper . 'Development.Shake.FilePath.takeBaseName') '?>' \\out -> do
+--     let src = 'Development.Shake.FilePath.replaceBaseName' out $ map toLower $ takeBaseName out
+--     'Development.Shake.writeFile'' . map toUpper =<< 'Development.Shake.readFile'' src
 -- @
 (?>) :: (FilePath -> Bool) -> (FilePath -> Action ()) -> Rules ()
 (?>) test act = rule $ \(File x) ->
@@ -141,10 +141,10 @@
 --   matched by more than one pattern. For the pattern rules, see '?=='.
 --
 -- @
---   \"*.asm.o\" '*>' \out -> do
---       let src = dropExtension out
---       'need' [src]
---       'system'' [\"as\",src,\"-o\",out]
+-- \"*.asm.o\" '*>' \\out -> do
+--     let src = 'Development.Shake.FilePath.dropExtension' out
+--     'need' [src]
+--     'Development.Shake.system'' [\"as\",src,\"-o\",out]
 -- @
 --
 --   To define a build system for multiple compiled languages, we recommend using @.asm.o@,
diff --git a/Development/Shake/Files.hs b/Development/Shake/Files.hs
--- a/Development/Shake/Files.hs
+++ b/Development/Shake/Files.hs
@@ -35,10 +35,12 @@
 -- | Define a rule for building multiple files at the same time.
 --   As an example, a single invokation of GHC produces both @.hi@ and @.o@ files:
 --
--- > ["*.o","*.hi"] *>> \[o,hi] -> do
--- >    let hs = replaceExtension o "hs"
--- >    need ... -- all files the .hs import
--- >    system' "ghc" ["-c",hs]
+-- @
+-- [\"*.o\",\"*.hi\"] '*>>' \\[o,hi] -> do
+--     let hs = 'Development.Shake.FilePath.replaceExtension' o \"hs\"
+--     'Development.Shake.need' ... -- all files the .hs import
+--     'Development.Shake.system'' \"ghc\" [\"-c\",hs]
+-- @
 --
 --   However, in practice, it's usually easier to define rules with '*>' and make the @.hi@ depend
 --   on the @.o@. When defining rules that build multiple files, all the 'FilePattern' values must
diff --git a/Development/Shake/Locks.hs b/Development/Shake/Locks.hs
--- a/Development/Shake/Locks.hs
+++ b/Development/Shake/Locks.hs
@@ -3,9 +3,11 @@
     Lock, newLock, withLock,
     Var, newVar, readVar, modifyVar, modifyVar_,
     Barrier, newBarrier, signalBarrier, waitBarrier,
+    Resource, newResource, acquireResource, releaseResource
     ) where
 
 import Control.Concurrent
+import Control.Monad
 
 
 ---------------------------------------------------------------------
@@ -57,3 +59,77 @@
 
 waitBarrier :: Barrier a -> IO a
 waitBarrier (Barrier x) = readMVar x
+
+
+---------------------------------------------------------------------
+-- RESOURCE
+
+-- | The type representing a finite resource, which multiple build actions should respect.
+--   Created with 'newResource' in the 'IO' monad before calling 'Development.Shake.shake',
+--   and used with 'Development.Shake.withResource' in the 'Development.Shake.Action' monad
+--   when defining rules.
+--
+--   As an example, only one set of calls to the Excel API can occur at one time, therefore
+--   Excel is a finite resource of quantity 1. You can write:
+--
+-- @
+-- do excel <- 'newResource' \"Excel\" 1
+--    'Development.Shake.shake' 'Development.Shake.shakeOptions'{'Development.Shake.shakeThreads'=2} $ do
+--        'Development.Shake.want' [\"a.xls\",\"b.xls\"]
+--        \"*.xls\" 'Development.Shake.*>' \\out ->
+--            'Development.Shake.withResource' excel 1 $
+--                'Development.Shake.system'' \"excel\" [out,...]
+--  @
+--
+--   Now the two calls to @excel@ will not happen in parallel. Using 'Resource'
+--   is better than 'MVar' as it will not block any other threads from executing.
+--
+--   As another example, calls to compilers are usually CPU bound but calls to linkers are usually
+--   disk bound. Running 8 linkers will often cause an 8 CPU system to grid to a halt. We can limit
+--   ourselves to 4 linkers with:
+--
+-- @
+-- do disk <- 'newResource' \"Disk\" 4
+--    'Development.Shake.shake' 'Development.Shake.shakeOptions'{'Development.Shake.shakeThreads'=8} $ do
+--        'Development.Shake.want' [show i 'Development.Shake.FilePath.<.>' \"exe\" | i <- [1..100]]
+--        \"*.exe\" 'Development.Shake.*>' \\out ->
+--            'Development.Shake.withResource' disk 1 $
+--                'Development.Shake.system'' \"ld\" [\"-o\",out,...]
+--        \"*.o\" 'Development.Shake.*>' \\out ->
+--            'Development.Shake.system'' \"cl\" [\"-o\",out,...]
+-- @
+data Resource = Resource String Int (Var (Int,[(Int,IO ())]))
+instance Show Resource where show (Resource name _ _) = "Resource " ++ name
+
+
+-- | Create a new finite resource, given a name (for error messages) and a quantity of the resource that exists.
+--   For an example see 'Resource'.
+newResource :: String -> Int -> IO Resource
+newResource name mx = do
+    when (mx < 0) $
+        error $ "You cannot create a resource named " ++ name ++ " with a negative quantity, you used " ++ show mx
+    var <- newVar (mx, [])
+    return $ Resource name mx var
+
+
+-- | Try to acquire a resource. Returns Nothing to indicate you have acquired with no blocking, or Just act to
+--   say after act completes (which will block) then you will have the resource.
+acquireResource :: Resource -> Int -> IO (Maybe (IO ()))
+acquireResource r@(Resource name mx var) want
+    | want < 0 = error $ "You cannot acquire a negative quantity of " ++ show r ++ ", requested " ++ show want
+    | want > mx = error $ "You cannot acquire more than " ++ show mx ++ " of " ++ show r ++ ", requested " ++ show want
+    | otherwise = modifyVar var $ \(available,waiting) ->
+        if want <= available then
+            return ((available - want, waiting), Nothing)
+        else do
+            bar <- newBarrier
+            return ((available, waiting ++ [(want,signalBarrier bar ())]), Just $ waitBarrier bar)
+
+
+-- | You should only ever releaseResource that you obtained with acquireResource.
+releaseResource :: Resource -> Int -> IO ()
+releaseResource (Resource name mx var) i = modifyVar_ var $ \(available,waiting) -> f (available+i) waiting
+    where
+        f i ((wi,wa):ws) | wi <= i = wa >> f (i-wi) ws
+                         | otherwise = do (i,ws) <- f i ws; return (i,(wi,wa):ws)
+        f i [] = return (i, [])
diff --git a/Development/Shake/Oracle.hs b/Development/Shake/Oracle.hs
--- a/Development/Shake/Oracle.hs
+++ b/Development/Shake/Oracle.hs
@@ -29,7 +29,7 @@
 -- > addOracle ["ghc"] $ return ["7.2.1"]
 -- > addOracle ["ghc-pkg","shake"] $ return ["1.0"]
 --
---   If a rule depends on the GHC version, it can then use @'getOracle' [\"ghc\"]@, and
+--   If a rule depends on the GHC version, it can then use @'askOracle' [\"ghc\"]@, and
 --   if the GHC version changes, the rule will rebuild. It is common for the value returned
 --   by 'askOracle' to be ignored.
 --
@@ -41,7 +41,7 @@
 --
 --   Actions passed to 'addOracle' will be run in every Shake execution they are required,
 --   their value will not be kept between runs. To get a similar behaviour using files, see
---   'alwaysRerun'.
+--   'Development.Shake.alwaysRerun'.
 addOracle :: [String] -> Action [String] -> Rules ()
 addOracle question act = rule $ \(Question q) ->
     if q == question then Just $ fmap Answer act else Nothing
diff --git a/Development/Shake/Rerun.hs b/Development/Shake/Rerun.hs
--- a/Development/Shake/Rerun.hs
+++ b/Development/Shake/Rerun.hs
@@ -29,10 +29,10 @@
 --   the environment. For example:
 --
 -- @
---   \"ghcVersion.txt\" '*>' \out -> do
---       'alwaysRerun'
---       (stdout,_) <- 'systemOutput' \"ghc\" [\"--version\"]
---       'writeFileChanged' out stdout
+-- \"ghcVersion.txt\" 'Development.Shake.*>' \\out -> do
+--     'alwaysRerun'
+--     (stdout,_) <- 'Development.Shake.systemOutput' \"ghc\" [\"--version\"]
+--     'Development.Shake.writeFileChanged' out stdout
 -- @
 alwaysRerun :: Action ()
 alwaysRerun = do Dirty _ <- apply1 $ AlwaysRerun (); return ()
diff --git a/Examples/Test/Resources.hs b/Examples/Test/Resources.hs
new file mode 100644
--- /dev/null
+++ b/Examples/Test/Resources.hs
@@ -0,0 +1,31 @@
+
+module Examples.Test.Resources(main) where
+
+import Development.Shake
+import Examples.Util
+import Control.Monad
+import Data.IORef
+
+
+main = do
+    let cap = 2
+
+    ref <- newIORef 0
+    res <- newResource "test" cap
+    shaken test $ \args obj -> do
+        want $ map obj ["file1.txt","file2.txt","file3.txt","file4.txt"]
+        obj "*.txt" *> \out ->
+            withResource res 1 $ do
+                old <- liftIO $ atomicModifyIORef ref $ \i -> (i+1,i)
+                when (old >= cap) $ error "Too many resources in use at one time"
+                liftIO $ sleep 0.1
+                liftIO $ atomicModifyIORef ref $ \i -> (i-1,i)
+                writeFile' out ""
+
+test build obj = do
+    build ["clean"]
+    build ["--threads2"]
+    build ["clean"]
+    build ["--threads4"]
+    build ["clean"]
+    build ["--threads10"]
diff --git a/Main.hs b/Main.hs
--- a/Main.hs
+++ b/Main.hs
@@ -14,6 +14,7 @@
 import qualified Examples.Test.Files as Files
 import qualified Examples.Test.FilePath as FilePath
 import qualified Examples.Test.Pool as Pool
+import qualified Examples.Test.Resources as Resources
 
 
 fakes = ["clean" * clean, "test" * test]
@@ -21,7 +22,7 @@
 
 mains = ["tar" * Tar.main, "self" * Self.main, "c" * C.main
         ,"basic1" * Basic1.main, "directory" * Directory.main, "errors" * Errors.main
-        ,"filepath" * FilePath.main, "files" * Files.main, "pool" * Pool.main]
+        ,"filepath" * FilePath.main, "files" * Files.main, "pool" * Pool.main, "resources" * Resources.main]
     where (*) = (,)
 
 
diff --git a/shake.cabal b/shake.cabal
--- a/shake.cabal
+++ b/shake.cabal
@@ -1,7 +1,7 @@
 cabal-version:      >= 1.6
 build-type:         Simple
 name:               shake
-version:            0.2.5
+version:            0.2.6
 license:            BSD3
 license-file:       LICENSE
 category:           Development
@@ -102,3 +102,4 @@
         Examples.Test.FilePath
         Examples.Test.Files
         Examples.Test.Pool
+        Examples.Test.Resources