See http://www.xfire.com/cms/stats/

Granted, it only counts people with xfire installed, but I doubt there is much of a selection bias (i.e., people that would never install xfire tend to play some other MMO that isn't listed).

Also interesting are all the games that come between WoW (#1 in hours played per day 413,790) and LoTRO (#20 - 10,007).

With sucha huge hours-played disparity this makes me wonder if the market is "ripe" for a stronger second-place contender or not.