Evolution RTS is a free an Open Source RTS game designed using the Spring Engine. It has been in development for over 5 years and receives updates often. Its gameplay is designed specifically to appeal to gamers of all skill levels. The gameplay is designed around the use of unit groups, and the mixture of your unit group often will determine how successful you are in battle, as opposed to churning out hundreds of the same unit continually.
The economy and teching system is easy to learn and use. The game itself supplies many tools that make managing your economy easier so that you can focus more on crushing your enemy than of playing simcity in your base. Units interact using realistic physics and varied terrain (you can actually micro units in order to dodge projectiles!).