Resident Evil 5 is great online in co-op, you don't even need a mic to communicate effectively. It also has the most entertaining lag I've ever seen.
Red Alert 3 is cool if you have a friend with a mic to co-ordinate attacks, but otherwise it just falls apart.
I love Unreal Tournament III online. It's a small community and nobody uses mods but it's still a great arcadey FPS. Especially if 4 friends with mics team up to tackle the campaign on the highest difficulty.
GTA4's online is good if you can get a decent game (stupid P2P is stupid, plus most of the online players are ten-year-olds).
Team Fortress 2, even the unpatched PS3 version, is so addictive.
LittleBigPlanet is one of my favourite online games. It's great to find a well-made community level, especially with friends. I haven't even created anything myself yet. I hope Team Creation comes out of Beta soon.
Via PS3, I can play my PSP Ad-Hoc games online. Phantasy Star Portable and Monster Hunter are great with others, but I've also had some hectic Dissidia matches and a couple games of WotL's co-op.
And finally, I got the Uncharted 2 online beta once it went public. It's a lot better than I expected. The single-character gameplay mechanics translate well into the online, and even though it takes a while to FIND a game (due to bad servers, not a lack of players, thousands play it) once ingame there's very little lag, and at least it isn't input lag. And the co-op modes are a blast.