I loved duo-ing Borderlands 1&2 (PC) with a friend. Had a lot of fun duo-ing Resident Evil 5; haven’t tried it on RE6 yet. If you like button-mashing action-RPGs, LotR:War in the North is okay.
Seriously, my girlfriend and I play it together and enjoy it a fair bit. That said some pretty hardcore, typically guy type shooters were listed as previously enjoyed games above. If those are the preferred flavor of games, forget my suggestion.
If Skylanders is of interest, some rumors are floating that on Black Friday Best Buy will have staters for $50 and a blowout deal on series 1/ last year’s figures which are a great deal for expanding the game on the cheap.
It’s easy to forget that the 360 has backwards compatibility, since it’s neither comprehensive nor fully working with all the titles it theoretically does support, but I believe titles like Baldur’s Gate: Dark Alliance, Justice League: Heroes, and the X-Men Legends games work fine and I can vouch for them being quite enjoyable in splitscreen.
Also, on a current gen note, Marvel Ultimate Alliance and sequel are both lovely as well.
All co-op games end to soon. Gears of War 3 while a lot of fun seemed a very short co-op game. Probably under 20 hours.
Borderlands or the Laura Croft ideas sound good.
We liked Halo ODST until we realized that for some reason the game allowed us to “jump ahead” in the plot and therefore we were never able to actually finish the game. We “touched” something that jumped us ahead in the plot.