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).

(this file is the Windows setup)