I'LL KILL YOU, YOU BLOODY CAMPER
On a sidenote, offline co-op games that I own: Gears of War, Rainbow 6 Vegas, Ghost Recon Advanced Warfighter, GRAW2, Halo3 (Halo1/2 also work for backwards compatibility). Now, these don't work so well if you don't like FPS/third person shooters. Otherwise, Guitar Hero II (though you need 2 guitars, but I found it worth the price), NHL 2008. I don't remember if Forza2 (realistic 'sim' driving game) has 2p or not, but I only have one wheel, so the point is moot. If you like JRPGs, Eternal Sonata is a neat little lighthearted one. There may be some others, but I forget.
Other offline shooters are Kane and Lynch, Earth Defense Force 2017 (both 3rd person shooters). Marvel Ultimate Alliance is apparently a solid brawler (correct me if it lacks 2p offline). There are some others, but they may not necessarily be worthwhile. I think Kameo and Perfect Dark Zero have it as well.
If you're into oldschool arcade beat-em-ups and are willing to get Live Arcade games, Streets of Rage 2, Golden Axe and TMNT are fun diversions. Aegis Wing is a free horizontal shoot-em-up. The arcade versions of Contra and Super Contra are also available. Ikaruga isn't out yet, but that one is going to be awesome (vertical shoot-em-up).