LLVM. It is a ridiculously well written and well documented codebase. The transformation/optimization framework is remarkable and surprisingly approachable. The compiler intermediate representation is easy to work with both in the emitted bitcode files and in llvm's internal data structures. I find it a joy to work with.

Its also cool due to all of the other great technologies that are taking advantage of it (Mono, Parrot, Adobe CS4, Apple)

I think once llvm and clang become mature enough, the FOSS community will completely supplant gcc with this technology.