What are some decent First Person Shooter multiplayer games on the PC?
Apart from: CS, CS:Source, Call Of Duty & Battlefield.
Rainbow six: vegas, Splinter Cell: spy vs merc. FEAR Combat(Its free, too)
Wolfenstein:Enemy Territory, it's free too :)
Day of Defeat Source is fun...
Control Monger is a lot of fun if you can find people to play with. It's empty most of the time.
Halo for PC is fun. It's pretty cheap these days, it's been around since 2003, but it's still fun as hell. The servers are quite populated.
Medal of Honor: Allied Assault is another good older one. It's a WWII first person shooter.
For free, you can download WarRock. It's a modern-combat game, kinda like BF2, with similar environments and weapons, but with a weapons purchasing system and game modes more like CS:S. It's fun.
Tremulous, I like it, its multiplayer, its free, how can it get much better
Medal of Honor ( i have allied assualt), Quake 2 and 3 ( yes it may be old but its fun as hell especially 3!!), quake 4(continuation of Q2), Doom3, Fear, and HalfLife 2 xD