I've been playing this recently - pretty unique idea, I must say I was rather surprised to see that a link wasn't posted here.

Gameplay Trailer:
http://www.youtube.com/watch?v=hJd9iCbpwuICourtesy of
Wikipedia:
World of Tanks is a 3D team-based massively multiplayer online game featuring historically accurate fighting vehicles from the 1930's to the 1950's. World of Tanks is a PvP-focused project that promises action and strategy based gameplay. In the release version, World of Tanks will include more than 150 armored vehicles from America, Germany, and the Soviet Union, carefully detailed with historical accuracy. All in all there will be around 500 vehicles available in the game. At the moment, World of Tanks features light, medium and heavy tanks, tank destroyers and self-propelled artillery and some well-known prototypes and low-volume vehicles. The game blends of action, strategy, simulation and MMORPG genres. The game is currently in the beta testing phase, and vehicles from Russia, Germany and USA are available for play.
World of Tanks is the first game of Wargaming.net studio to be released under "Play 4 Free" model, which will see the game released for free with revenue generated from micropayments in the form of Gold. The release of the game is planned for Q3/Q4 2010.
Game modes:
The players in World of Tanks will have a chance to take part in three primary types of battles: random battles, team battles and clan battles. In random battles players cannot choose in which team out of two possible they can play. On the contrary, in clan battles gamers decide to fight for a particular clan. Each team can have no more than 30 players in it; therefore the maximum possible number of combat units at the battlefield is 60. The goal of each team is to defeat all of the enemies and try to lose as few allied tanks as possible.
The basic mode of the game will be the team deathmatch, therefore teamwork and mutual cooperation will be very important if you want to be successful in the game. The game will also have a capture-the-base mode, with tanks having to occupy the enemy’s base for a quicker win. Developers also plan to design capture-the-flag mode and the domination mode with players having to hold certain control points.
Along with this there will apparently be support for clans and the like:
World of Tanks will also include a never-ending campaign with Clan Wars unfolding on the Global Map. The Global Map with a particular part of the Earth will be introduced and split into certain districts. Clans (or blocks of them) will be fighting for these provinces. In the course of time other continents will be added so there’ll be a chance to take part in battles for superiority somewhere in the desert areas of Africa or tropical jungles of Asia. To form up a clan one should gather a team of at least 15 players with up to 100 participants. Joining a clan will open new opportunities for the player – he’ll get access to better tanks, new perks and benefits. However, those willing to stay away from clan membership will be free to act as Mercenaries that could be recruited by clans on special occasions. There will be a place for political and non-political methods of waging war and gaining control and power over their own and foreign territories on the global world map. Diplomacy will play an important part in Clan Wars as there would be various options available – non-aggression pacts, mutual aid treaties, intelligence sharing or free pass for allied armies. Another feature which will be added once the Global Campaign is completed will be introducing historical missions – series of battles set up on the maps close to their historical prototypes where only specific models of tanks will be allowed.
When you participate in battles, depending on your performance (damage, kills, spotting) and whether you won or not will factor into how much experience and money you receive. Money is used to repair your tank after the battle, buy more ammunition, train your crew, buy new upgrades and finally: buy new Tanks. Experience is used to research upgrades for tanks and to research new tanks, you can also set it so that all accumulated experience goes into crew training.
Tech Trees (note: not all tanks are released yet):
*
American *
Soviet *
GermanThe game (as mentioned) is currently in 'closed' Beta, it's entirely PvP focused and it will be Free to Play - you will be able to buy "premium" vehicles - which are currently pretty mediocre, plus you'll be able to pay for a subscription which increases XP and money gains, transfer experience and buy in-game money.
If you want a key for the Beta, see the post HERE - I personally used the Shattered Crystal link for one.As a side note, I have no idea where the servers are hosted for this, though I'm Australian and I average about 250 ping to the server and it still plays wonderfully.