I'd like to mention a new MMO project by Wargaming.net World of Tanks. The game is very promising. It can't be called a true historical game but the tank models corresponds to their historical prototypes (machines of 30-50s). Details can be found here: http://www.worldoftanks.com/
Also a trailer from youtube: http://www.youtube.com/watch?v=ax2ilBa4Oiw

Gameplay video here: http://www.youtube.com/watch?v=lk3RLFrckZ0

Some more gameplay videos http://www.worldoftanks.com/news/three_gameplay_movies

World tanks is a team based massively multiplayer online dedicated to armored warfare. Participate in epic tank battles of World War II with the other cowboys of steel worldwide. Its arsenal includes more than 150 armored U.S., Germany and the Soviet Union, with a carefully detailed historical accuracy.

Well, there are number of the variety of the games which are easily available into the market. They include a variety of game types, representing many types of video games. But a massively multi player on-line game is a multi player video game which is capable of supporting hundreds or thousands of players simultaneously.