Combat Mission: Beyond Overlord (CMBO West Front) is a bit dated now but still lots of fun.
Combat Mission: Barbarossa to Berlin (CMBB Eastern Front) upgrades the graphics and tweaks the game.
The just released Combat Mission: Afrika Corps (CMAK Italy, N. Africa, West Front?) is the latest and last for this engine. AK has a few minor bugs probably only noticable by the grognards, I'm sure these will be patched up. There are plans for a new game engine for future games.
You might be able to find CMBO and CMBB special editions (extra scenarios and mods) in stores (I saw CMBO-SE in best buy, CMBB-SE at EB). CMAK is only available by mail (as were the other titles when first released).
All games are playable via tcp/ip or pbem. There is a thriving mod and scenario creation community.
Demos are available for all 3 games of the Combat Mission series at
battlefront.com.