A while ago I discovered Minecraft4k, a java demo written by Notch fitting into a 4kb object file. He used software raycasting to rasterize a large 3d array of voxels. The original demo can be played here as an applet:
One awesome thing about these two demos is that all of the textures are synthesized proceduraly for space saving reasons.
Seeing his work, I got a strong urge to port the code to C++ and SDL.
Grab the source code here: