Normally I try to only post the really geeky stuff on The Coding Monkey, but this is so damn useful, that I have to cross post it.  There is a new beta site called Zamzar, which is a free site for performing any of 150 different conversion types.  You see the complete list here.  Need to convert a Word document to PDF but don't have Acrobat?  Done.  Have some AC3 files you want to move over to MP3?  No problem.  Go check it out.