The crew behind the iPodLinux Project have ported Linux to the iPods hardware, at least first, second and third generation models. (iPodLinux doesnt run on fourth gen, mini or photo models yet.)
What do you do with iPod Linux?
From what weve seen, viewing photos and playing games that look quite a bit like Tetris, Pong and Asteroids are the most popular options. There are quite a few more programs in the process of being ported.
Its not a permanent change. A quick reset and your iPod will drop iPodLinux for the regular Apple software.