Magic: Online. Yes, it's the electronic version of the collectible card game.
That game is like the polar opposite of World of Warcraft. It's inherently anti-social, in that anytime you are interacting with someone you are actively trying to prevent them from winning, whereas Warcraft is all about cooperation to achieve a goal.
In Warcraft, if you are a skilled player, you'll be getting tells within minutes of logging in asking if you are available to do this or that.
In Magic Online, nobody cares if you log in or not.
On the one hand, it's a very expensive game because you have the buy collectible "cards" to expand the plays you can do. On the other hand, it works like an investment- I could easily sell the cards in my account for at least $2,000 if I got into a financial pinch of some sort.
It really is a good game with a lot of depth and complexity. The game it's based on was designed by a mathematician, and there are literally tens of thousands of different spells (cards). I do miss the cooperative aspects of World of Warcraft however.