1 Haskell Wiki Search
Google indexes the Haskell Wiki much better than the internal engine: Add your search term to: Google site:haskell.org
You can use the search box at the top of the page to just search the Haskell wiki. Note that words of 3 characters or less are not currently indexed. Alternatively as suggested by Daniil Elovkov on Haskell Cafe you can add site:haskell.org/haskellwiki to a Google search.
2 Hoogle : Library Search
You can use Hoogle to search for a function name or type signature such as map or (a -> b) -> [a] -> [b].
3 Hayoo! : Search for function names etc
You can also use Hayoo! to search for function names, module names, signatures and function descriptions.
4 Google : Search *.haskell.org/*
The Search haskell.org link provides a Google search restricted to *.haskell.org/*. This is wider than just the Haskell wiki - it includes the GHC documentation, the online report etc.