Pirate Galaxy is a free to play sci-fi, ship-based MMORPG.  It can be played through a browser (from Windows) or via the downloadable java client.

In the game, you take command of your own spacechip. You must pilot your ship over the surface of different planets in search of Cryonite Crystals. To obtain these minerals, you must destroy enemy Mantis ships and seize the cargo they are carrying. Other human players will be there, competing or collaborating with you in the hunt for minerals.