If you haven't played EVE Online yet, you might want to check it out. It's a single-server MMORPG run by Iceland developer CCP Games, and has had a consistently growing population for six years now.
The game recently broke 300,000 subscriptions worldwide, which when compared with the titan of MMOs may not seem like a whole lot, but when you think about the fact that they're all on one server it's quite a different story.
Head over for a free trial of EVE Online if you want to get in on the action! You can easily upgrade to a paid subscription after the trial runs its course.