Netrek

Team up to battle with and against people around the world. As a player, you captain different starships to engage enemy ships, bomb armies and invade planets in order to expand your team’s space empire and conquer the universe. The ultimate goal being to wipeout the enemy race. Think that sounds easy? Think again, since you are always teaming up with different people as well as playing against different people, there is always a challenge.

http://playnetrek.org/