Though you're going to look at the new thread, I'm going to put in my two cents here, too.
All of Mario's suggestions are excellent. If you have a Gameboy and have some friends with Gameboys, I definitely recommend Legend of Zelda: Four Swords (though I own it and I played the whole way through on one player and still had a blast). Super Monkeyball is really fun, and while I haven't played the second one, it's probably an improvement on the first. Sonic Mega Collection (compilation of old sonic games on one disc) is cool. Mario Kart: Double Dash and Kirby Air Ride are both fun racing games, especially for multiplayer (F-Zero GX has a mediocre multiplayer, but is excellent and challenging as a one player game). Super Mario Sunshine is a great game, too.