If Guitar Hero I, II, IIe:RtE & III no longer provide you with enough of a challenge, try your hand at Rock Band, which released today.
There are a number of great tracks on this game, including Metallica's Enter Sandman and BOC's Don't Fear the Reaper (more cowbell, anyone?)
The equipment is pretty sweet too, featuring not only a guitar controller, but a drum controller and microphone as well.
This wikipedia article has more details and some pretty decent pictures: http://en.wikipedia.org/wiki/Rock_Band_(video_game)
Good luck finding a copy though, if you didn't prepay or reserve yours, everywhere I've checked so far doesnt even have enough to cover the preorders.