I've used it for parts of two commercial projects (not counting side projects). One was a small parser -- C#'s parser options were uninspiring while FParsec (based on Haskell's Parsec) was great. The other was to simulate a client-server algorithm we were developing under different network situations. F#'s ability to express algorithms concisely was a real win there.

Side projects have mostly been stats/data based: using type providers to mash together different data sources, with visualization via FSharp.Charting.