The Simple Lifeforms blog went through a recent DFC Intelligence report on the most commercially successful MMOs and pulled out some interesting points. World of Warcraft took home the most revenue for a single game in 2008 with an estimated $500M+ and all MMOs combined (worldwide) earned an estimated $1,875M.The top 10 earners list for 2008 included only one game that launched in the 1990s and that MMO was Lineage I. The majority of the top 10 list also featured MMOs of the fantasy genre, which probably won't come as a surprise.
Simple Lifeforms classified four primary ways that make MMOs money: retail, subscription, virtual goods sales (aka RMT), and hybrid models. While the retail/subscription model is generally considered the tried and true way to run your MMO business by most of the big publishers, you might find it interesting that only half of the top 10 list is comprised of these types of MMOs.
An unfortunate drawback of the DFC Intelligence report is that the revenue figures for the nine titles other than WoW have very large spreads. We see they made more than $150M and less than $500M but aren't given anything more specific than that.