Electric Sheep is a free open source code screensaver run by thousands of computers all around the world.
When your screensavers starts running, it creates an electric sheep which is sent to a central server were it is mixed with other electric sheeps and they'll create a robotic dream.
Furthermore, you can rate the electric sheeps, and the more votes an electric sheep has, the more times it appears in different computers, If you create some sheeps and they become popular, they will interbreed with the rest of the flock and produce children. By mutating and crossing the genomes, variations of your sheep will appear and themselves evolve.
If you like Blade Runner, this is a must have for your Mac.