Dyson is the most fun I've had with a downloadable PC game in, oh, weeks.
Developed by Rudolf Kremers and Alex May, in essence it's a real-time strategy game made simple. There's only one type of unit, two types of building, no explicit resource management, and the units select targets for themselves.
In theory, Dyson is about mining robots who are colonising an asteroid field, but the art is abstract enough to let you believe whatever you want about the totally absent storyline. Units grow on trees, and fly around their home asteroid in quick orbits. You can sacrifice 15 units to plant a new tree, up to whatever the level-designated maximum is for trees per planet. To progress you'll need to conquer nearby asteroids and clear out infestations of other mining-robot tribes.
The art style is clean and attractive, and the simple visuals allow for swarms of thousands of units to appear on-screen without any noticeable slowdown. The ambient music neither distinguishes itself nor detracts from the experience.
I love Dyson to bits, but the current version is still a working prototype and there's a lot of work still to be done. The simplicity of the game means that, as far as I can tell, the same basic strategy will carry you to victory on every map, although that doesn't make using it any less satisfying. Only five scenarios are included with the game, which are all much like each other. The specific level layouts are procedurally (and randomly) generated, which means there's considerable replayability to be had, but it also means that some configurations can be significantly easier than others.
Dyson is a small download, clocking in at about 17 MB, and is available for free through the developers' website. It's a finalist in the 2009 Independent Games Festival and if tactical gameplay or procedurally generated content are your type of thing you should definitely give it a shot.