It's also worth mentioning that Star Wars: Knights of the Old Republic, while not an MMO, ranked highly for the week as well (third on Steam and fourth on D2D). BioWare announced a special $10 deal for KotOR at PAX last week for fans of their upcoming MMO, Star Wars: The Old Republic. People buying KotOR through D2D will receive a 25% discount on SWTOR when it releases. Speaking of D2D, you should keep your eye on their special Five Year Anniversary page because next Monday ten RPG/MMOs will be going on sale for $5.00 each!
Update: Thanks for the fix, cb!