The first game that comes to mind is Wario Ware: Smooth Moves. Now, it is very fun, and all in all a great game. But if you've played the other Wario Ware games, this one will seem a little toned down in comparison. They could've done a lot more with it, like most Wii games, but it's still damn fun. You might be able to find it cheap now, as well.
Guitar Hero III: Legends of Rock is a right blast, but you'd need to buy a 2nd guitar and that would cost quite a lot. It may not be worth buying a 2nd guitar just for the chance to play it with a friend either, it'd be more worthwhile if your friend(s) owend the game too.
Mario & Sonic at the Olympic Games is actually pretty fun too. It got mediocre reviews, but multiplayer is where it's at, though you'll have to unlock a lot of the games in single player first. If you liked the Wii Sports package, you'll probably like this as well.
I'd suggest steering clear of Mario Party 8 & RedSteel though, they're really not as fun as they might seem.
Wait for Mario Kart & Super Smash Bros. Brawl if you want some real fun multiplayer games.