Custom-TF
Custom-TF is a multiplayer Quake modification that provides a similar experience to Team Fortress (and Team Fortress Classic), but allows players to customize their soldiers however they want - imagine a sniper with a backup weapon that shoots laser guided rocket clusters, or a spy who sneaks behind enemy lines and sacrifices his enemies to summon powerful demons. See Custom-TF.
ProZaC-TF Coop
ProZaC-TF Coop lets you run through all the single player maps you've got with friends, souped up monsters, and all the custom stuff you are used to. See ProZaC-TF Coop.
Quake 4 Fortress
Quake 4 Fortress brings the original Team Fortress to Quake 4. See Quake 4 Fortress.
Improved Custom-TF
Improved Custom-TF aimed to improve Custom-TF's balance with several new ideas and features. It is now unmaintained.