There is this fascinating story of what happened when a “disease” was released into the Worlds of Warcraft game as an experiment. Apparently it got out of control because of unexpected behavior on the part of some of the players. What’s fascinating is that epidemiologists are now looking at using MMOGs as simulation labs to study disease outbreaks. The benefit is that it adds the unexpected human element that pure simulations can’t match. You can also read about it here.
Another MMOG is Eve Online. Apparently participating in the gaming world economy is the key part of Eve. So much so that they have now had to hire real world economist to try to figure out what is happening to the Eve world economy. Apparently inflation is a big concern there.