This game isn't an RTS, but I don't know where else to put it. Line Rider is a game where you draw on a canvas of unlimited capacity, and have a little man with a sled ride on it. It is THE BEST online Flash game ever! (note that you NEED Flash 9 for it to run. 'Adobe.com') First you can start out with him just riding striaght lines and jumping off of ramps and cliffs and things, but when you get more advanced at drawing things for him, you can create loop-de-loops and such. (Take a look at YouTube.com, and search Line Rider. You should come up with a few stunt videos and such. I love this game. I hope you will too!
P.S. Search Line Rider in Google, and it should be the first result that pops up. If not, use the second one.