Amplitude

This is an easily overlooked game, yet it is one of the most addicting and original games I have ever played. It is the sequel to frequency. Your goal is to unlock songs by unlocking tracks by playing through the game. It's multiplayer is as intense as it gets since the game leaves little room for error.
Anyone with a PS2 I at least recommend renting this game because you really have no idea what you are missing out on. I just found out about it last week and have been hooked since.