Command and Conquer Generals Zero Hour
This one is jam packed with new stuff, from the allies of red alert one, to better balance, to amphbious units, to even a few maps to play (download seperatley).
This latest release allows you to play with all three soviet sides along with a decent portion of the japanese side.
Here's a tiny taste of the action. This initial demo for you lets you play as either the soviets from red alert 1 or 2. Simply follow the instructions and the sides will be available to you in skirmish.