Serious Sam 3: BFE - Blood by the pound
I knew this game would probably be much like its predecessor, and under normal circumstances, I would not have bought it. But this being one of the few titles made for Linux from the very first days, I decided to cash in USD39.90 and help both the game developers and Steam in their Linux endeavor.
Serious Sam 3 cannot be compared to the likes of Call of Duty or aforementioned ArmA II, because its purpose is not to be a realistic war simulation. Instead, the game is oriented toward fun, with lots and lots of gore in it. Your task, so to speak, is to slaughter as many aliens as possible. Maps are usually large and open, allowing you to go wild with your weapons, as well as allowing huge hordes of enemies to charge your way. Of course, there's a plot and all that, but they are secondary to your mass-murder fun.