Everybody plays
L4D and with that quick play system we always have a chance of having players on it. It's a solid game, problem is L4D can get boring rather quickly. Even in VS mode if you played long enough you can almost play in yer sleep.
Commercial Game http://www.youtube.com/watch?v=xWqpibO7tR0Unreal Tournament still has a fairly dedicated fanbase. But more for 2004 than UT3. They're both rather old though, and the folks that play tend to stick to their favorite already established servers. Commercial Game
http://www.youtube.com/watch?v=16wJwrcUYF8 <- UT2004
http://www.youtube.com/watch?v=UVEytKj7UDc <- UT3
Eternal Silence is a free very good quality HL2 space mod sorta like a mix between Freespace and Star wars battlefront. It's also one of my favorite games. But it's not very well known so there is rarely more than 2 servers that have players in it. (Not counting some modded server that is hacked to appear always near full when it isn't) There is also a bit of a learning curve, which means newer players will often get destroyed repeatedly til they learn the ropes. A tutorial and steam achievements are in the works though. I go by [DFC]Alpha One on that instead of Greiger. If I had to pick one game, this would be it.
HL2Modhttp://www.youtube.com/watch?v=FwAHcQckI7kSmashball is a free fast paced soccer/football game with guns. Its a bit different. And not a bad game. Problem is the community is less than friendly and there is also a steep learning curve which again, gets newbies destroyed quickly.
HL2Modhttp://www.youtube.com/watch?v=UXKigCcJkxY (though the player isn't very good)
Age of Chivalry is a free midevil FPS, with a class based objective capturing gameplay. Though the gamemode itself is more in the map scripting. Theres CTF style maps and TDM style maps and KOTH style maps. Classes are things like longbowman, Man at Arms, Knight,,, Crossbowman, Cavalier, so on so fourth. It pushes the source engine's melee system to the limit. Downside is the Source engine Melee system isn't the most reliable in the world. And the game is balanced for friendly fire on, which is enough to turn some folks off of it when you get people who don't care about the location of allies when swinging their weapons. FF can be turned off, but then class balance goes out the window.
HL2Modhttp://www.youtube.com/watch?v=JtSKWWoo7aA