Team Fortress 2. It's an online only game with nine classes. It's (obviously) team based. Team RED vs Team BLU. There are a lot of maps and different playstyles.
The classes are Scout, Soldier, Pyro, Medic, Heavy (weapon's guy), Demo Man, Sniper, Engineer, and Spy. Every so often they release a patch that will introduce achievements which, when you do enough, will unlock one of three new weapons for the class that the patch was given. For example, the Heavy has a gatling gun (named Sasha) that does a lot of damage and fires fast, but he's very slow. So to balance out that everyone could run away from him, they created the Natasha. It's another gatling gun, but it has lower damage, though To balance this out, it has a slowing effect on it.