SpaceTrace is a completely free real-time strategy game set in a simulated galaxy.
It has quite a steep learning curve, but once you master the battle mechanics, you are rewarded with a game that encourages creative and cooperative gameplay.
SpaceTrace offers a unique time management experience: the game mostly plays at a casual pace, but in a tense battle situation quick reflexes can mean everything.
It is a sandbox-style game, that is most fun when played cooperatively.
ST is essentially a classical browser game, but unlike others it has a real-time engine and real 3-dimensional vector arithmetics.