Haskell is a polymorphicly typed, lazy, purely functional programming language. It can be used in a variety of applications like robot control, web servers, artificial intelligence, etc.
Real world applications include: (See Also: http://www.haskell.org/practice.html)
The glorious Glasgow Haskell Compilation System (GHC) - http://www.haskell.org/ghc
David's advanced revision control system (very new) - http://www.abridgegame.org/darcs/
A web server - http://www.haskell.org/~simonmar/bib.html
Happy - YACC for Haskell - http://www.haskell.org/happy
HaXml - utilities for Haskell and XML - http://www.cs.york.ac.uk/fp/HaXml/