[silentbicycle]

Besides the Glasgow Haskell Compiler (which probably doesn't count, for your purposes, but it's a good example of a large and complex program), the best example is probably Darcs (http://darcs.net/), a distributed version control system with a powerful patch composition system.

There are further answers from the Haskell wiki here: http://haskell.org/haskellwiki/Haskell_in_practice