Here's one I enjoyed back in the day:

http://www.totalsims.com/

They have three sims: Warbirds (WWII aerial combat), Dawn of Aces (WWI), and Armored Assault (WWII tank combat). I was thinking they had rolled them all into one game but it appears they are still separate....Armored Assault may not be super realistic, but it's a LOT of fun, plus there are a few aircraft available which makes it all the more interesting. Conversely, IIRC, Warbirds has a few tanks and ground vehicles available, so there is some overlap.

There are offline versions of the games, but they are intended for online use and a paid account. I haven't played in years, too busy with RC planes and full-scale flight simulators ( http://sabrelinersim.com ).

Matt