I'm surprised no-one has mentioned Anarchy Online. http://www.anarchy-online.com/
Yes, it's been around for a while, and it ain't perfect, but it's a fun game, and you can even try it out for free. In fact, I've been playing for over 18 months on a free account, and thoroughly enjoy it.
The basic game is the free bit, with the Shadowlands, Notum Wars, and Alien Invasion expansions being subscription only.
There's another expansion coming out in about 6 months or so, called Lost Eden.
Anyway, if any of you do decided to try it out, I can often be found hanging out in Borealis on one of my many toons ;-)
Cambo lvl 158 Adventurer
Cambots lvl 100 Engineer
Cambureau lvl 47 Bureaucrat
Cambonk lvl 18 Enforcer
Cambosol lvl 54 Soldier