If you folks haven’t seen this, it’s worth checking out:
You can compile and run it on Linux (if you’re running Linux there’s a strong possibility it is already installed or easily available from whatever flavor of package management your distro uses) and there is a binary available for Mac OS X too. Unfortunately no Windows for you poor folks out there.
Seriously though, this is a fantastic set of visualizations with many based on mathematical or physical concepts, and it is all open source. There’s a bunch of OpenGL code as well as plenty of beautiful 2D stuff.
Plus–lest I forget to mention it–there is a 3D bouncing cow screensaver.
Oh, and it’s C code, which is part of the point of me mentioning it here…