If you like Fighters, pick up BLazblu continuum shift. Even if you listen to Iceglow and get a 360 instead of PS3, it's on both systems so you wont' miss out. Definately my favorite 2D fighter to date.

Same thing with Boarder Lands. Great co-op, though only for 2 people if you play off one system, and it's not something you could switch on and off with like a fighting game.

For something just on PS3, Modnation Racers is a solid choice. The customization of racers is great, so you could have a lot of fun with more then just the racing if you're into the custom designing.

For 360, I'll join in and say there are much better multiplayer shooters then Halo, such as CoD. Now it works on PS3 online decently, though in the end if you are willing to pay for the online, you'd be better off on 360 with it. There are also the kick ass Gears of War games for some amazing co-op experience.

And while it still has it's issues, both Army of Two games are solid and fun Co-Op, available for either system.

As for 64, Conkers. I actually had more fun with the multiplayer on that then with Golden Eye. However skip the XBox port if you get an XBox, because they botched the Multiplayer badly.

And for PS2, Soul Calibur 2 is an awesome fighting game, still my favorite in the series.

ANd while you're at it get Dark Cloud 2, not for the Multiplayer or anything(I think there was some kind of Multiplayer option, but I never used it), but because that game kicks ass.