Unison: http://www.cis.upenn.edu/~bcpierce/unison/index.html
In fact I think this is the only software that I know is written in a functional language that I use on a regular basis. I've always figured that said very good things about OCaml. (Not that I've ever gotten around to learning OCaml, mind you.)
But, most FP languages have only recently started gaining real traction outside academia. There are some great tools written in e.g. Haskell: pandoc (a document converter), Hakyll (a static site generator), Darcs (version management), xmonad (an X11 window manager), git-annex, and multiple web frameworks (Yesod, Snap, Happstack).
In recent years, some important foundational libraries have been developed (bytestring, text, vector, repa, conduit/enumerator, etc.). I can imagine that it's pretty much the same in OCaml and F#.
The language is really elegant, but I'm not sure I ever saw a real-life use for it until today.