360 multiplayer.

Veldrin

I just recently picked up a 360 for myself and my brother and grabbed a copy of Dead Rising and Gears of War along with it. What I am looking for though, is any recommendations for good multiplayer (vs, co-op) games that are currently out that we can both play together.
I've glanced around briefly in EB, and a few titles caught my eye (GRAW, Splinter Cell:DA, Tony Hawk: Project 8 ), but I'd rather get a few opinions before I go jumping in the deep end.

I'm open to suggestions from any genre, although if anyone knows of any decent mech-style games then that'd be great.

Samphis

Lego Star Wars II(Retail)
Heavy Weapon (Arcade)
Assault Heroes (Arcade)
Mutant Storm (Arcade)
Marvel Alliance (Retail)

Coming soon:
Alien Homind (Arcade)
Castle Crashers (Arcade)
Monster Madness (Retail - think top-down 3D remake of Zombies Ate my Neighbors)
Crackdown (Retail)

The only real choices for Mech games is pretty much Chromehounds, but it has no local co-op or versus. It is, however, pretty much distilled awesome if you can get a group going.

Lach

Rainbow Six Vegas and another nod for Assault Heroes

Dr_Keenbean

Lego Star Wars 2
Contra
Perfect Dark Zero has co-op and is dirt cheap (for good reason)

Ganzidan

Perfect Dark Zero is good. If you like a pretty basic FPS, in the frame that Goldeneye for the N64 is. Plus they now have some of the Goldeneye maps for download.

Top Spin 2 for all your tennis needs, can do versus as well as online.

Lego Star Wars II, has co-op and is just generally a good fun game. Legos, Star Wars, what isn't to like?

Those all have co-op and/or multiplayer.

Lost Planet, I'm enjoying since it is exactly what it is, a third person arcade shooter. Haven't gone online yet but it does have it, sadly no co-op.

edit: beat by the doc.

Damn am I in the mood for some TS2 and LP.