Team Fortress 2
Team Fortress 2 is a long standing free to play first person shooter, in the resent years Valve has upgraded the game to allow for custom map mods, weapon mods etc. making it one of the most versatile and creative shooters out there.
Players can pick from 9 different classes each time they spawn, all play differently and all of them have TONS of cool stuff to unlock such as weapons, outfits, secondary items etc.
If you haven't played TF2 yet you owe it to yourself to give it a try.