Wolfenstein: Enemy Territory

Enemy Territory was always the game at LAN parties where the serious players migrated once the casual rounds finished. Class roles are clear, objectives give the match direction, and the team communication that emerges from a good round of Radar or Goldrush is the kind of thing people still talk about an hour after the match ends. It's free, the dedicated server setup is well-documented, and the map design on the classic rotation is still some of the best objective-based work in the genre. The one real limitation for modern groups is there are no bots. If you can't fill both teams with real players, you need another game. With full teams, it's outstanding.