World of Tanks is the only one I know of, but that is in multiplayer beta. I don't know if it will have a single player campaign.
World of Tanks is out. I have been playing it for a year now, it is no sim, but if you like tanks it is a ton of fun. Especially when you get to the big gun tanks. I just wasted some enemy heavies with the 105 on my Jagdpanther 8-)
I would say give World of Tanks (WoT) a try, it is completely free, has simple controls, sometimes annoying grinds for upgrades, and a steep learning curve that rewards intelligent playing. It is no Call of Duty or Battlefield crap.