Bruce Sterling Woodcock of MMOGChart has been tracking the subscriber numbers for a variety of different MMOs for years; version 19.0 of his data was released about a week ago, providing numbers up to January 2006 in a long-awaited update. The chart's coverage isn't perfect; geographic information would be especially useful, but is near impossible to find, and the site explains the accuracy behind the data (there are differences between active subscribers, cumulative subscribers and boxes sold). However, it's interesting to see what this snapshot of the current MMO world is like.
The graph above shows the largest MMOs, those with over 120,000 subscribers. The green line leaping towards infinity is World of Warcraft -- the yellow and red curves, declining slowly, are Lineage and Lineage II respectively. Many of the others stop in 2005, so it's hard to get an accurate up-to-date picture, but the light blue line representing Runescape seems to be finally taking off, cresting the 500,000 barrier -- an impressive feat for a game started by a couple of students.