Ask Justify: multiplayer computer games?

So videogames have become increasingly social among my friends for a while now.  I'm trying to expand that and wanted to poll my readers for your favorites.

Suggestions, ideas, or success stories?  If it has two or more players and high replay value I'm interested.

Preferences:

1. Something that supports groups (4+)
2. Games with low system requirements, if only because its usually cheaper
3. Cooperative to competitive (tends to be more socially positive), but the option for competition is nice
4. Something fairly easy to learn and not restricted to first person shooters

Past examples:

• Modern Warfare 2 of course but not much beyond the hardcore gamer friends
• Diablo 2 - was great for groups and a lot of fun but burned out on this
• Gamecube is cheap, seats 4, and has some nice games
• Modded old xbox also 4 player and can be outfitted with MAME for some nostalgic gaming nights
• Wii Tennis entertained *really* well for like two weeks