Battlefield 1942 is a popular online FPS set in WW II. Its a very fun game which allows you to pilot airplanes, drive tanks and jeeps and even aircraft carriers, battleships and subs. As with most FPS games released nowadays, BF1942 can be customized (or modded) by anyone with enough time and energy.
WW II was 50 years ago, but this mos I ran across is set a long time ago, in a galaxy far, far away. Yup, Galactic Conquest is set in the Star Wars universe (that is, until Lucas realizes they exist) and it looks like you get drive all sorts of cool things, including AT-ATs and Snowspeeders. If only Star Wars Galaxies had this. Sweet! It also looks decent. I haven’t played it since I don’t have BF1942, but this mod looks really good.