The Enemy Territory: Quake Wars demo is available at their official site Here

I probably don't have to tell most of you that this is pretty exciting and super cool. You can even play this multiplayer vs friends and it is free. The demo contains a full versino of the "valley" battleground. This is set in Yosemite California which is a suprisingly hip and ambitious location. There all kinds of terrains in this zone as well as side missions to do. For a full run down of information about "valley" read this guide

Quake Wars is a team based shooting game like Day of Defeat. Some players are regular infantry with machine guns, but others take on more tactical roles such as building automated turrets, repairing buildings, healing the wounded, or being a stealthy assassin. The teams work towards a number of objectives on each different map.

The demo also features Enemy Terrirories: Quake Wars Bots. This means you can play single player against computer controlled opponants, and experience the games highly praised AIs. This is useful tactically if you want to learn the map intimately before jumping right into a competetive game.