Well... I don't know if you own any of these games or are even looking for these as answers but...
Halo: Reach - If you don't own this and have an Xbox 360, get it. It pretty much has the whole shabang when it comes to online.
Battlefield: Bad Co. 2 - Class shooter kind of similar to CoD in that its a more modern military FPS. Pretty much a more advanced CoD... Best way to describe it I think.
Left 4 Dead/ Left 4 Dead 2 - One of the best multiplayer experiences on the 360 IMO probably only second to Halo. Pretty much a survival point A to point B game but its really fun and if you get into it, you can lose a lot of time into them. Its best played online with buddies, but you can search for strangers or just play offline and have bots be your team. They also have a versus mode where you play as zombies and L4D2 has special scenario playlists.
Borderlands - Similar to Left 4 Dead in that its a coopertive shooter, but this game is more of an RPG and really best played when you have buddies to play with instead of strangers online. They have a nasty habit of screwing with your game, i.e. dropping modded guns or activating quests you don't have, messing with your quest log. This game, you'd really only want to play with people you know or just by yourself.
BioShock 2 - Not the greatest game on earth, but its pretty solid. The Multiplayer is kind of like CoDs where you pick your weapons and powers and customize your character and play different match settings. When you find your niche, it can be really enjoyable. Plus, you can pick it up for around $10. Multiplayer may be dead though.
The Orange Box (Team Fortress 2) - TF2 for the PC may be the definitive version, but that doesn't make the 360's any less fun. Its a class shooter with a cartoony look to it. Each class has their job to help benefit the team as a whole complete their objective.
Monday Night Combat - If you like TF2, this is really similar to it. Think of it like a TF2 meets Tower Defense kind of game. Its on the XBLA for 800 msp (or $10 in human speak)I believe.
Resident Evil 5 - Not a great horror game, but if you like Gears of War, you may enjoy this game. It's only 2 player Co-op so it's suggested you have a buddy play this with you, if not may want to pass.
Splinter Cell: Conviction - 2 Player co-op stealth game... It's Splinter Cell. About the same as RE5, no buddy, you may want to pass.
Portal 2 - 2 Player co-op puzzle game. If you're looking for multiplayer only, this game may entertain you for a while and it's really good, but not a whole lot of staying power TBH. You may get a good 16 hours if you play it extensively but otherwise, you may be better off with another game if you're looking for a good time sink.
Red Dead Redemption - Great sandbox game which transitions to a great open-world multiplayer game. It also has matchmaking playlists if you just wanna kill people.
Assassin's Creed: Brotherhood - Probably the most unique multiplayer experience you can get on the system. Kind of like a hide n' seek game, where you have a target to find and kill while (depending on the game type) a player is trying to find and kill you.
I just talked about the multiplayer aspect of these games. A lot of them have great single player campaigns as well. Hope this helps you out
