Allegiance is a game who was made by Microsoft, but they released the source code.
Since then, fans keep working on it... (looks like FSO huh ?)
It's some Tactical/Technical/Whatever-al space combat game
You can only play it in multiplayer (there are just a few solo training missions)
It works with a server-game system : a server hosts a game, which has defined settings and so on.
In those games, 2 to 6 Teams with different factions (afaik there is 6 or 7) fight each others
This include : Mining He3 from asteroids with Miner vessels (they're automated drones), building bases with the constructors, and of course fighting others.
But it's not just random dogfighting : you have to place probes in order to detect ennemies, provide nanite or turret support, even ram bombers on a bomber run to make em go to the enemy base faster !
There are also teleporters which are very important.
Well that's a quite complex and fun game...
I though some people here would be interested in it, then it's here :
www.freeallegiance.org