MGS4 Brilliant even if you havent played the others.
Drakes Fortune - Excellant game. Bit like tomb raider with a male lead instead.
GTA IV if you dont have it for another console.
Hotshots Golf Out Of Bounds for fun and if you like golf.
Resistance Fall Of Man - Great game but gets some bad reviews because its always compaired against Gears Of War.
Motostorm is not bad if your looking for a racing game to.
Sid miers civilization if you dont have if for another console.
Pro Evolution soccer 2008 if you dont have on Pc or another console.
Games to look forward to are -
Killzone 2 (not till early next year now),
SOCOM Confrontation (out soon),
Resistance 2 (Looks amazing tbh)
Dead Space, Siren Blood curse, Resisdent Evil 5 and many many more comming soon. (Probably missed load's to)
Good luck with your choices. Theres alot more avalible now than there was at launch and also theres a lot more to look forward to