The GNU Compiler Collection (once upon a time known as the GNU C Compiler), GCC for short. Vendor branch can befound at http://svn.netlabs.org/libc/browser/vendor/gcc/, current code at http://svn.netlabs.org/libc/browser/trunk/src/gcc/ (will later be moved to http://svn.netlabs.org/libc/browser/trunk/gcc/).

The GCC home page can be found here: ​http://gcc.gnu.org/ And if you wonder what Wikipedia has to say you better go ​here.