The recently-revealed Twilight Princess exploit allows all sorts of unsigned code to run natively on the Wii. Homebrew developers are already starting to take advantage, releasing a bunch of interesting and/or useful unauthorized programs for Nintendo's little white box. Among them are Linux for Wii, MP3 player, SNES emulator, and Wii Tetris (read the full article for links to details and downloads.)
Nintendo has not reacted to the exploit. However, given their recent aggressive pursuit of game pirates, you can expect a PSP-style battle of the firmwares to start any day now.