I'm not a big-time gamer, but on occasion I get the urge to blow stuff up. I'm also not exactly rich. It would be silly for me to spend a lot of money on a console, or even $50 on an FPS that I probably won't play every day. It is for this reason that I really, really like OpenArena.OpenArena is based on the ioquake3 engine. Ioquake3 is, in turn, based on GPL'd Quake3 code form id Software. While ioquake3 is little more than a framework for games, OpenArena takes it and packages the engine along with other GPL licensed content (level maps, weapons, character models and such) to give you a ready-to-play Quake3 clone, and for free!
Maybe in the near future, I'll set up an OpenArena server so we can duke it out. Otherwise, look for me (ax0n) playing on some of the Capture The Flag servers.