Kinda amazed that nobody here has picked up on this one yet. Awesome tank game where you can drive a tank from WWII (and a bit into the 50's) and blow up other tanks from the era. They plan to have Japan, Italy, Britain, France, USA, Germany, and USSR tanks but only USSR, Germany and USA (minus USA TDs for now) are available so far. It is technically 'closed beta' but you just register and they send a serial a few days later.
I have been playing it for a few weeks and have yet to get bored of taking my IS and firing 122mm rounds into the rears of King Tigers. Other awesome things are the ricochets, I have had rounds where it took them 75 shots to whittle down my tank because 3/4 of their guns were too weak to penetrate my armour or they were shooting from the wrong angle. The game keeps a huge amount of reality while not going completely overboard.....oh, and it has the Maus.
And a Russian one that remains awesome despite the language barrier.
http://www.worldoftanks.com/