There are plenty of games that simulate the Army, some are based during WW2, some are based in current times. Red Orchestra is one that comes to mind, you can drive tanks (awesomely fun btw). Battlefield 1942 and Battlefield 2 obviously come to mind. You can drive a ton of vehicles. Operation Flashpoint: Cold War Crisis is another awesome one. Came out a year before BF1942 did, and did things better to boot.
If your asking for a free game then no, your options are seriously limited. There is America's Army, which is free. But there are no controllable vehicles, yet. The next patch due out soon, will allow you to control a Hummer. That's about it though.
There's always Warrock I suppose?






