CCP Games' Massive Title, Eve Online, now boasts 100,000
subscribers. Though there are many games with more users Eve Online is a very different title, set inside ships in the depths of space. They currently hold the record for most concurrent users, set at 23,178 simultaneous users on a single server. From the article: "To help accommodate its growing population, CCP will complete a hardware overhaul, allowing the game to handle more users, expand its universe, and run smoother."
Ethic, over at Kill Ten Rats, has been writing about Eve a lot lately. His posts cover intergalactic war
and courier missions
, and might give you a sense of what gameplay is like. If you're interested in that sort of thing.