Socom 2
THUG
007 EoN
Resident Evil Outbreak
Madden 05
Final fantasy xi(pay per month though)
Off the top my head, those are the games that gave me tons of fun online. This is assuming you have friends who play online with you. For instance me and my friend play 007 EoN but no one but us are in the lobbies sometimes. Resident Evil is a love it or hate it game, most who played and hated it did so offline. Playing over the phone and hearing one of your 3 other friends get scared cuz their phone is dying is classic, even more so when you see them get carried up into the sky by a moth, you laugh your ass off and get on them when you hit the lobby after the game.
Same with socom, except using the headset is fun on that. Hearing one of your friend's voice cut off because they got blow up by a gernade while you flew back and now hear a high pitch noise...thats the true meaning of playing online!
A few games to lookout for i would think Killzone, Goldeneye(i think its gonna be online), and OPM, new socom 2 maps finally here!