Currently on sale as part of a Humble Bundle:
Natural Selection 2
Sanctum 2
Magicka + DLC
Orcs Must Die! 2: Complete Pack
Garry's Mod
Serious Sam 3: BF3
I've got Natural Selection 2 and it is a fun game, a FPS and RTS combination with the Commander building stations for Marines that try to wipe out the enemy Aliens' nest. Not a very big community, but I'm sure it'd be a blast with a small party using Teamspeak.
All of these can be yours for the low price of $4.15 (price subject to change).
https://www.humblebundle.com/