Arctuvian Gambit is the Free Online Space Strategy Game.
Arctuvian Gambit is turn based strategy, that is quick to start and easy to learn.
Arctuvian Gambit is browser based, there is nothing to install.

Play on Windows OS

Windows 7, Windows Vista, or
Windows XP + .NET 3.0 or better. Get it  here.
1152 x 864 display or better
FireFox 3, IE 7, or better
Play on Mac OS X

Mac OS X
FireFox 3, IE 7, Safari 3 or better
Silverlight 2.0 Get it  here.