The Python interpreter is some pretty clean and educational C code, IMHO. Why don't you join python-dev, find some bugs in the issue tracker and start hacking? http://www.python.org/dev/contributing/
I will do just that. First glance at the issue tracker gives me the impression that most of it's over my head, but this post has me inspired: http://news.ycombinator.com/item?id=1048202 . Time to learn! Thank you!