An impressive JavaScript demo in 1kb (view the source) (p01.org)

The really clever thing about this demo is how it packs source code as a PNG. Because PNG is a lossless image compressor, it's fine to pack arbitrary data in it. It can then be unpacked with getImageData and then executed with eval.

If you want to see what the unpacked version looks like, it's here: http://www.p01.org/releases/MATRAKA/matraka.unpacked.html