A "mod" for HalfLife in which there are two teams: terrorists and counter-terrorists. This game is intensely popular. If you ever go into an Internet cafe type place chances are you'll see people playing it. I never really got into it. It's ok I guess, but the crowd who play it tend to take it a bit too seriously for my liking. Some people are insane about how they play this game. It pretty much validated the "mod" model of games though - HalfLife just won't die, even though it's a really old game by industry standards, and that's mostly because of CounterStrike.