Believe it or not, it was a flash game. You can't play it online anymore, though you can find it for download somewhere I'm sure.

It was called "Obliterate Everything 2" - it was just a small game about space battles, with fairly simple mechanics. But the amount of depth it got from that was so absurd. The difficulty curve was a bit wacky, and I remember talking at length to my friends about the various game design principles I'd learned from observing and judging it.