LittleBigPlanet is a must for multiplayer. Also, as far as shooters go, everyone else suggested some great ones, but I would also recommend trying Battlefield: Bad Company. Single player is fun, and the multiplayer is pretty fun. It's a little different than most traditional "kill everyone but you/your team" multiplayer. It's more of an attack/defend style of game. Worth a rental at least, in my opinion.
If you want a great party game, Buzz! Quiz TV is really cool. Not only do you have the 4 player format, but you also have the online mode where you can actually play as a team of up to 4 people on your PS3 against anyone else's team over the internet.
Another great multiplayer game, if you have the cash, is Rock Band. It's great with 4 people, but it's still just as fun with 2.