Open source first-person shooters

One area where free software is sort of lacking is first-person shooters. There are a few, but not many. I'll touch on what I think are the three most notable examples.

1. OpenArena
OpenArena is a free and open source shooter based on Quake III Arena. They look different (OpenArena looks nicer) but play about the same. It also has a decent amount of active multiplayer servers. It runs pretty well on most computers too, a bonus if you're installing GNU/Linux on an older computer and want to play games on it, for example.

2. Freedoom
Freedoom is more of a novelty. If you still want to play Doom but want an open source implementation, this is your best bet. Freedoom is pretty much the same as Doom; however, it's a work in progress, so there are still missing parts. You also need to use your own Doom engine, like PrDoom.

3. Nexuiz
Honestly, I haven't played too much Nexuiz, so I can't say too much about it. It runs on a heavily modified open source Quake engine. It's a pretty nice-looking game, and there were always people online whenever I tried it.

OpenArena gameplay
Freedoom gameplay
Nexuiz gameplay