Paintball II is a mod where teamwork isn't an option -- it's a requirement.  With support for up to 4 teams; ctf, single flag ctf, siege, deathmatch, and elimination modes; 7 different guns; a totally new palette; lots of new models/gfx/textures; paint grenades and smoke bombs; an infinite number of possible flag combinations; tons of custom maps; funname support; and a bunch of server-side options, Paintball2 is one of, if not the, most advanced mods/TC's for Quake2.  It can be a bit overwhelming for newbies, but hang in there, 'cuz it's really fun once you get the hang of it.  It's all I really play online anymore :)

Check it out at the Digital Paint page.  Get it.  Play it.  Learn it.  Join a clan.  Have fun! :)