Quote: AssaultCube is a FREE multiplayer first-person shooter game, based on the CUBE engine.

Taking place in realistic environments, with fast, arcade gameplay, it's addictive and fun!

With efficient bandwidth usage, it's low-latency and can even run over a 56Kbps connection.
It's tiny too, weighing in at a lightweight 40 MB package available for Windows, Mac and Linux.
On the correct settings, it can even run on old hardware (Pentium III and above).

Quote:    * It's FREE.
    * Open source, available under a zlib-like open source license.
    * Low latency, it can even run across a 56Kbps connection!
    * Light-weight size, only 40MB to download, plus additional maps average 20KB each!
    * With the correct settings, it can run on old hardware (Pentium III and above).
    * Officially runs on most major systems, and maybe even some non-major ones?
    * Has a built in, in-game map editor to help players create their own maps and allows for
      co-operative editmode in realtime with others!
    * Features a single-player bot system.
    * Supports recording of your game by the "demo" system.
    * Contains 12 multiplayer game modes, including: Deathmatch, Survivor, Capture the Flag, Hunt the Flag, Keep the Flag, Pistol Frenzy, Last Swiss Standing & One-Shot One-Kill (plus team versions of these modes).
    * Comes pre-packaged with 19 different maps!