Game of Life
I got very bored in a far-too-easy networking class and ended up coding a very memory efficient version of Conway's Game of Life.
Of note, it uses bit-twiddling/bit-packing to access individual bits on byte-accessible memory for the gameboard, and it uses a sort of buffer-thing to further reduce memory usage, rather than push temporary changes into an entire other temporary game board. I just made it up as I went along. The idea behind being this memory efficient is so that I can plop this on a microcontroller (possibly CSH's Big Infosys?) and just let 'er rip on a LED matrix :-)
Source is up at http://github.com/clockfort/Life/