All real time stratagy fans should check out the Supreme Commander demo out. Here's the link to Gamespot's download (you need to be registered). Although I got better speeds from a torrent.
It weighs in at just over 1GB, and features some of the single player campaign and skirmish (one map).
I suppose the main attraction of this game is the scale of it. Some of the maps are 80km x 80km in size. You can also zoom all the way out to see the entire battlefield. Makes a change from Battle for Middle-Earth and the late Command and Conquer games which just felt too zoomed in.
Highly recommended. We'll be reviewing the final game on Gamercast when we get our hands on a final copy (probably for the episode on the 18th).