GCC (GNU Compiler Collection)
GNU's reference compiler suite — Richard Stallman, 1987. Supports C, C++, Objective-C, Fortran, Ada, D, Go. Compiles the Linux kernel and 99 % of Linux distros.
Monolithic architecture (frontend, middle, backend tightly coupled). GCC 4.x (2005+) added Graphite optimisation and GIMPLE/RTL as internal representations. Latest versions have a plugin API but not in the Clang style.
Competitor: Clang/LLVM (more modern architecture, better error messages), MSVC (Windows). GCC retains pole position on Linux for historical reasons + Linux kernel's GCC-specific extensions.