Good Games for LAN Parties?

An Anonymous Coward asks: "I'm trying to figure out what games are good for LAN parties and especially what games don't require a CD after installation or at least allow multi-player games with only one CD. I'd like to have friends over to play over a LAN without everyone needing to own the same game. Starcraft will let you do this but I couldn't find much info about other games so I thought I'd turn to Slashdot. Which games do you like for LAN parties and why, and especially which games allow a group of people to play if only one owns the CD?"