256b.com is an archive of 256-byte executable demos. Yes: 256-byte executables. These are actual Windows executables that draw something interesting, and are only 256 bytes long.
It's hard to imagine how 256 bytes of compiled code can do anything significant, but take a look at Tube, which produces a real-time dynamic pseudo-3D effect:
Another interesting entry is Self Disassembler, which disassembles its own code, and prints it to the screen. This is a good example of a quine, a program that when run prints its own text. The Quine Page has a large number of these programs.
Both 256-byte demos and quines are examples of geek challenges that twist the rules of coding to get a particular effect, much like the polyglot programs I wrote about earlier this month.