If you own one of the big three systems and have any interest in pretending to be a slammin bangin rock god, then you owe it -owe it I say- to yourself to go out and buy Rock Band by EA Games. This thing rocks like nobody's business.
In the bundle pack (which is gigantic, so clean out your car before you get it) you get a guitar, a microphone, and a set of drums, along with the game of course. All this stuff is pretty much full size too- it's not chintzy or embarrassing to play around with at all.
The other night I had a 4 player game going- lead, singer, drums, and bass (I used my GH3 guitar for the bass- works perfectly with RB). I had everybody in the same room but you can get a game going over the net too. I haven't tried that one yet but it sounds really, really cool.
In my opinion, Rock Band's head and shoulders above Guitar Hero. We were up till about 2 in the am playing 'Learn to Fly' and 'Creep' over and over. Un-freaking-believably fun.