MMOCrunch has started a monthly feature based on data from new site VOIG that shows subscription details for various different MMOs. What that boils down to is that they'll be reporting each month on the most subscribed-to games, position changes in the rankings, and basically which company gets bragging rights for that month.
The first report examines the data for October this year. The top 5 MMOs by subscription in October were World of Warcraft with 26.03% (surprise!), Second Life with 18.86%, Guild Wars at 12.6%, Knight Online with 11.9%, and Dofus on 9.73%. WoW, SL and Guild Wars I could probably have given you, but I had not realized that Knight Online and Dofus were so popular.
Other interesting tidbits: Lord of the Rings Online hit 300,000 subscriptions, EVE Online overtook EverQuest II and Vanguard grew a little, but still isn't doing too well. Assuming that the VOIG data is reasonably accurate (something that MMOCrunch themselves say they wanted to be sure of, and watched the site for a few months first before running the article) this should be an interesting feature to watch in coming months.