Since everyone else just seems to tell you to get a console, I'd thought I'd actually list a few good PC games that are actually fun to play multiplayer on the same screen. Yes, they do exist. It's a great platform for the task, and if people started actually taking it seriously as such, we could probably see more games like this.

Worms Armageddon (already been mentioned a few times)

Heboris (great customizable two player Tetris game)
http://tetrisconcept.com/wiki/index.php?title=Heboris

Super Mario War (very fun party game where the goal is to stomp on the other player's heads)
http://smw.72dpiarmy.com/

Stepmania (DDR style game that can take all sorts of songs)
http://www.stepmania.com/

Atomic Bomberman (8 player Bomberman on the PC)

You Don't Know Jack (fun and wacky non-trivia game, huge series of them)

Also, try looking at the list of simultaneous player games at Home of the Underdogs. There are a LOT of games. (http://www.the-underdogs.info/multi.php?sort=SHS)