Another RTS I speak highly of is Warzone 2100. You can get it for free, legally, if you don't mind playing without the extra video clips. I own the original and still use the newer version because they updated everything. It still looks like it did when it came out (think PS1 era, I don't remember the original year it came out), but the textures are smoother and the multiplayer is a lot better. A lot more maps and a large amount of new things to research and customize. It's a really small game in size since they took out the videos, but you may want to watch some of them on Youtube or something.
note: this is not warez and is completely legal since Pumpkin Studios (the makers of the game) released it as freeware.
Here is the link:
http://wz2100.net/