Here are the ones I have/don't have/never played/etc
-Planetside 2 [never played]
-Team Fortress 2 [have, but haven't played much, aka no experience]
-Counter Strike: GO [don't have, I got the CS Source though... need to redownload]
-Quake (I think there's a Free Quake Online?) [never played ...]
-Star Wars Jedi Knight: Jedi Academy (I believe it's this one. It has arena style pvp multiplayer) [don't have]
-Minecraft [don't have]
-Unreal Tournament [had... need to dig it up again, or some free download somewhere]
-League of Legends? 10 people at a time. Not sure if this would work. [never played]
-Half Life 2 Deathmatch [have but never played]
-Battlefield? (I have Bad Company 2 and BF3) [ditto]
That's all I can think of off the top of my head.