There are several known forks of GCC. If you maintain or know of a fork, please list it here. In general the GCC community prefers that forks be integrated into the compiler proper, or at least that development happen in the GCC svn repository.
Vulncheck is a patch to add taint-style static analysis to gcc.
GCC-XML lets you generate an XML representation of C++ source.
llvm-gcc Connects the LLVM optimizer and code generator to the GCC front-ends, providing link-time optimization and better code generation in some cases.