from ars technica

As online games grow, the need to properly secure these virtual worlds becomes more important.

Massively multiplayer online games (MMOG)s, particularly of the role-playing variety (MMORPGs), have been growing for years, and while a great deal of that growth since 2004 is solely attributable to WoW, games like Runescape, EVE Online, and Second Life have established their own solid game bases north of 250,000 subscribers, while plenty of other games (Lord of the Rings Online, Conan, Pirates of the Burning Sea) have seen respectable if not overwhelming results of their own. (There's also some wacky game named Dofus at around 450,000 subscribers; it's both French, and Flash-based, but has still managed to garner a significant base for itself. Go figure.). As data from SirBruce's MMOGChart.com shows,the total number of active MMO subscribers has grown tremendously. 

MMOGChart, for those of you who aren't familiar with it, is probably the single best source