A small scripting language, possible using bytecode.
A C compiler.
An internationalization (i18n) library.
An implementation of the C++ STL for C.
An operating system.
A portable build system.
A "modern" editor (MVC pattern).
A game maker.
An RPG.
An IDE with integrated SVN support.
C programming resources:
GNU C Function and Macro Index -- glibc reference manual
The C Book -- nice online learner guide
Current ISO draft standard
CCAN -- new CPAN like open source library repository
3 (different) GNU debugger tutorials: #1 -- #2 -- #3
cpwiki -- our wiki on sourceforge