Garageband: OK, so it's not really a game, but Garageband is still plenty of fun — you can wail out on a guitar (or keytar), drum kit, keyboard (or keytar!), or bass (or bass keytar!), and build layers of music up into a masterpiece. You can even record your crooning with the iPad 2's inbuilt microphone — there's almost no limit to what you can achieve, although the lack of T-Pain autotune is a bit disappointing.
Find Garageband here on the iTunes App Store.