Watcom C - they're open sourcing the Watcom compiler! As of 8/30/01 when I wrote this it's not ready for public consumption yet, however.
Dev-C++ - a great free IDE that uses MinGW as its compiler. It works "right out of the box", and I'm very happy with it.
And of course, don't forget GCC, etc. (which you already have if you're running Unix) from GNU as well as the Cygwin port of it all. Links on my Linux page