Teamfortress is THE BEST addon for quake. It incorparates skill and teamwork into a fantastic multiplayer game which sees 2 team Blue and Red battle to capture each others flag. There are also 9 'classes' for each team, they are special men which can do different things, like the sniper who has a sniper-rifle whichs kills in 1 hit. The 9 classes are:

[1] Scout
[2] Sniper
[3] Demoman
[4] Soldier
[5] Pyro
[6] Medic
[7] Heavy Weapons Guy
[8] Spy
[9] Engineer
[0] Random PC