A week or two ago I was exploring Linux Mint's new Software Manager (Mint 11) and went to the most downloaded games. I found a game that looked good, called SauerBraten. I downloaded it and it was installed, no difficulties, thanks to the software manager.
I made some online research on it and it is a first-person-shooter, it's free and cross-platform, since it uses OpenGL Graphics, and the latter are very nice. They remind me of Team Fortress 2 graphics. The game is also very small, not even 500 MB.
The game boots almost instantly, the online servers work the first time, everything looks good. For the first time, I decided to play against bots, since it also features bot matches, and you can even specify the difficulty and game mode and map. I tried it and it was a bit hard to get used to, but thanks to the very detailed Options menu I got there.
I tried it online and the most popular modes are Insta CTF (Capture the flag with a one-hit KO Sniper), Regen Capture (you have some bases and you have to defend them and attack other team's bases and the goal is to have the maximum bases as possible) and Insta GIB (Deadmatch with one-hit KO Sniper). These are actually the only modes I play because there are about 10 servers with enough people to make it fun, and all these 10 servers have one of these 3 modes. This is not a big issue though, as these modes are the funniest.
SauerBraten includes a map editor and some default maps are original, others are from Counter-Strike and others from Quake. This is pretty cool, as the level editor is very easy to use.
For a free game and cross-platform I have to give this a 10 out of 10. It has some issues such as you can't crouch and the menus are quite non-user-friendly.