Anyone who happens to own a Gamecube, Gameboy Advance and cube-to-gba link cable -- I realize that even on a global scale this is only a handful of people -- absolutely must track down a copy of the cube version of Rayman 3 and unlock the "Mad Trax" mini-game. I'm not sure why Ubisoft went to all the trouble of developing this cube-exclusive addition to the game, but it's an absolutely ingenious piece of game design.
One player uses the GBA in a mode that looks and plays very similarly to Tetris. But this mode is actually an overhead view of a race course, and the dropping tetris-pieces are hunks of road that you use to assemble a giant floating highway (while dodging giant floating meteors). Meanwhile, the other player drives over this highway in an F-Zero styled 3D racer. Together, you try to guide the car to each checkpoint before time runs out.
It's one of the most fast-paced, incredibly addicitive co-op games I've ever played! And it's tucked away in such an obscure and unlikely place. If you are one of the few people with all the necessary hardware, I definitely suggest you check it out.