There were tons but by now most are dead in terms of multiplayer.
The hidden is amazing, the game is about a subject who escaped prison. He is invisible (like 90% invisibility, at cirtain light and angle you can sorta barely see the outline), he can jump very high stick to walls, even trow objects for physics kill. So like 8 solders with guns are deployed to kill him while the hidden must kill all of the soldiers. Tons of fun.
Battle grounds 2 was amazing too, similar to Medival Warfare but during the civil war in america, was tons of fun but now has become organized clans and line shooting (more historically accurate but boring in my opinion).
Zombie Panic - Another amazing multiplayer mod, starts off with 1 infected and however many survivors, the survivors must use planks to hammer in doors as well as furniture to block pathways or entrances, find various guns and survive the swarm of zombies, there are other levels where survivors must get to a certain point (from A to B) to be rescused, also tons of fun. This mod is actually stil fairly active if I am not mistaken.
Dystopia
Age of chivalry