Quake Wars, the followup to the Enemy Territory mod for Wolfenstein, is releasing in October, and a free 1-level multiplayer demo is going to be released monday!

http://community.enemyterritory.com/index.php?q=node/124

Apparently the demo will not only allow online play on the official servers, but also offline against bots, and running your own demo server.

For the uninitiated, ETQW is part of the "Strogg War" quake storyline, preceding the events in Quake 2. The game is set during the Strogg invasion of Earth, and players can choose to be part of either the Human or Strogg forces, each with unique units and vehicles. Massive team objective based FPS is the name of the game, but with the Quake flair for badass weapons and vehicles.

The requirements are surprisingly modest for the game, so I think most everyone in AT should be able to try it out: