Team Fortress 2
Team Fortress 2 is a class based action First Person Shooter. It was originally a commercial game but has been released as free to play. The classes in TF2 are as follows: Scout, Soldier, Pyro, Demoman, Heavy, Engineer, Medic, Sniper, Spy. Each class has a very distinct way of playing, for example as the engineer you can build turrets or as the spy you can disguise yourself as the opponent team.
TF2 features extremely high end graphics and production values not found in most free to play games. There is a large player base very dedicated to this game. A must have game if you did not purchase it when it was commercial.