It's a linux community project in the first place, but there are also versions for the poor users that don't run linux, and it's been in development for a long time: here's Tesseract 1.0: http://tesseract.gg
It's free and open source of course, and it's a twitch shooter, and the cool thing is that you can edit entire maps collaboratively with other players, even in-game. All of that with full 3D, dynamic lighting, ambient occlusion, and whatever graphical trickery tickles your fancy. And it's REALLY easy to do, there's almost no learning curve whatsoever.
It comes with CTF and Instagib modes for those who like that more that DM, and it goes a step further than Xonotic in terms of map design and collaboration.
Now drop everything and jump on it...