I'm going to put in a shameless blurb about an FOSS game that I play. It's called Tremulous, and it's available for GNU/Linux, Macintosh, and Windows operating systems. I've even heard of a few people running it on BSD.
There is no bot support in this game, so it's multiplayer only. There are two opposing teams; humans and aliens. Each team is vastly different than the other.
The human team for example is fairly straight forward. They are like any other human team in other games. They have ranged weapons much of which we have today such as a rifle, shotgun, chaigun, etc... however they also have a few futuristic weapons as well. I'll let you discover those. They have to buy these weapons in the same fashion as counter strike meaning you must get kills to get better equipment.
The alien team on the other hand do not have any ranged attacks save for one type of alien. Technically there are two, but you have to get so close with the other it's not much of a ranged attack. The alien team doesn't have weapons in the conventional sense. Instead they have what are called classes, and they evolve from the lower classes into a higher class. Just like the humans the aliens must also get kills inorder to evolve into a higher class. One thing to note is each alien class has different abilities such as walking on walls, or bouncing off them, or the ability to jump likee Superman. Then there are those which are just plain big.
Something I haven't mentioned yet is that each team has what can be termd as a builder class although for the humans it's just a matter of getting a tool instead of a gun. Each team is capable of constructing a base which is vital to the survival of their teams. These bases provide a strategy element to the game. You can use them as just a base of operations or you can set them up in a way as to give your team an advantage over the other. Mind you I haven't said anything about the structures, but each team gets vastly different structures which do completely different things.
I'm going to stop short here, and let you guys check it out for yourselfs. Just be aware that there are still things which I haven't told you about such as stages, so be sure to ask about them.