Originally posted by: Nothinman
The MS compilers are free, but there's no GUI for them. I also believe that MS released 'express' versions of some of their VS.Net stuff for free, not sure what languages are supported though.
gcc runs on pretty much every OS out there, Bloodshed DevC++ is a free Windows IDE that uses the mingw port of gcc.
And as Dave says, if this is for a class or work make sure you talk to your professor or boss becaue they'll probably require a specific set of tools.