1) Mario Kart DS - This game is a no brainer. Everyone who has a DS and a couple of friends must buy this game. I've played this game with 2 to 8 people and I can tell you that there is no better feeling than kicking your friends ass with a red shell right before they cross the finish line and taking the trophy from them. This graphics are nice and clean, the controls are tight and most important the game is fun. This game is will have everyone yelling and screaming with excitment. Having a DS without Mario Kart DS is a sin.
2) Bomberman Land Touch! - What could be more fun than blowing up stuff? Maybe blowing up your friends might be more fun. This game starts of slow so everyone can get power ups and plan their strategy. However, the game quickly speeds up and everyone is running around the two screens trying to kill everyone. At the same time your trying not to die by accident or by being blown up. If the time runs out the game quickly starts covering up the screen with blocks. If you are touched by a block you die. So you can imagine how intense this game can get. I've played this game with 2 to 4 players. Even when I was at Vegas my friends would play this instead of gambling. This game is a great game to add to your multiplayer experience.
3) Clubhouse Games - Four player bowling or bullshit (Nintendo gave it a kiddy name) are some of the great games in this package. This game is good, however this game is not for everyone or anytime. My friends and I only played this game when we felt like thinking. This game is a great multiplayer game, but you must be in the thinking mood.
4) Tetris DS - Just look at the screen. Tetris DS is the only game that I know of that has a 10 multiplayer option! This is also the only game that I've had a girl kick my ass over and over again! If you like video games you will love this version.
5) Pacman VS - I've never played the game, but you can make sure I'll go buy it. Thanks to Cryomancer.
What other games am I missing?