http://ageofempiresonline.com/

Hey guys, I just tested this game out. A friend of mine says its a lot of fun
you can build your empire and have to do some missions a la command and conquer
and apart of this coop missions are also avaiable

Its a free version, but you also can upgrade to premium by 25 EUR
so just test it, then we may can play together once :)

Greetz
Kinky