World Of Tanks is a free-to-play MMO with tanks. Wargaming.net explains: "The game offers fast random battles featuring various modes, historic mini-campaigns, tourneys and epic Clan Wars on a global map, where mighty clans fight for hundreds of provinces employing grand strategy, diplomacy and economic power. Besides individual tank-ace skills, WORLD OF TANKS requires team work and a fine balance between light and heavy armor." Great news, because there's no kind of game that cannot be improved by the addition of tanks. Also, because it's going to subvert normal MMO jargon when someone says "we need a tank for this group." There's a trailer below, and it's quite the thing. Ha.
The game is due for release in the Autumn.