All games are anyway. There are some good free online games though. You just need to find them.
Quite a number of them weren’t always free, like Anarchy Online and Enemy Territory, the later being built upon a commercial game. And the others are more or less futile attempts of certain states to enlist more cannon fodder (America's Army). On a side note, I never got how a game where you are almost certain to get shot every second round is a good advertisement for joining the army...