I'll second the people that mentioned Anarchy Online. I played for a while at release, and have gone back to it a couple times over the years. Now that they've released the basic addition as a free download, I've had it installed non-stop. Someone mentioned earlier that the free account is a one-year subscription, but while they talked that way originally, they've never made good on it, I've been on a free account for well over that at this point.