Saturn Bomberman [sega Saturn]- With 10-player support, and 4-player online support, it is definitely a fun game to play at big parties.
Faceball 2000 [Game Boy]- The only FPS on the, and probably the only 3D game on the Gameboy, along with 16-player link cable games with only 1 game cart, it is definitely the most powerful multiplayer game ever.
Death Tank Zwei [sega Saturn]- A 7-player "Worms" clone that's better than the game it's trying to imitate. Big, flashing lights, and a huge number of weapons, it's a fun little game to play if you get bored at a party.
Mario Kart DS [Nintendo DS]- Quite possibly the best racing game ever made, with 8-player local and 4-player online races on a portable console, I played this for years with my friends on the bus ride to school.