There's never been all that much, apparently due in part to the difficulty in developing for the GC (compared to, say, the Xbox).
There's stuff like GC Linux (which IMHO is a waste of time, proof-of-concept novelty, like most games console linux distros), and then there's a SNES emulator, if you're into that sort of thing. For the most part though, there's nothing on the GC homebrew scene that you couldn't get an equivalent (and probably better) PC version.
I've found that the best thing to do with my GC is to use it to play the handful of platform-exclusive titles that are any good. And given that you can get most of them on a '4 for £20' at Gamestation these days, there's no real need to resort to the powers of your mod chip
