There's Navy Field. It's free until you hit some level or other. You can just keep making new "characters" when you approach it to keep the game free.

Of course, it's probably not the type of MMO you were thinking of. You command a ship, join a battle, and battle against another fleet.

Check it out, though, if you get bored. http://www.navyfield.com/

What is it they say about silence being golden?