Hoogle is a Haskell API search engine, which allows you to search the Haskell libraries on Stackage by either function name, or by approximate type signature.
Example searches:
map
(a -> b) -> [a] -> [b]
Ord a => [a] -> [a]
Data.Set.insert
+bytestring concat
Enter your own search at the top of the page.
The Hoogle manual contains more details, including further details on search queries, how to install Hoogle as a command line application and how to integrate Hoogle with Firefox/Emacs/Vim etc.
I am very interested in any feedback you may have. Please email me, or add an entry to my bug tracker.