Awesome.
Everyone's mentioned the good multiplayer games, but as for co-op, there's not so much:
Rogue Squadron 3 - All of Rogue Squadron 2 is available in this game as two-player co-op, which is ace.
Mario Kart: DD - Easily the worst MK of the series unfortunately, but not all that bad for a quick laugh with a mate in co-op I suppose.
Tales of Symphonia - Excellent RPG, and the battle sections are real-time and co-op. Does require you and your friends to be RPG fans though, because most of the time they'll be watching you play through the story.
Final Fantasy: Crystal Chronicles - Requires a GBA and connecting wire for everybody that wants to play, so it's unlikely you'll have the equipment required. Which is a shame, because it's a fantastic game in multiplayer, and a good laugh, albeit not the most pick-up-and-play.
That's all I can really think of co-op wise I'm afriad but if any of those interest you, do give them a try.