Let me start off by saying that this is in my top ten favorite games of all time. Darwinia's a cool retro-style game about a digital world populated by little people called Darwinians, who are under siege by a dangerous computer virus. Under the instructions of Darwinia's creator, you have to help get rid of the virus and restore Darwinia to it's fully-funtional self.
The whole time playing I thought I had the ending figured out, I thought it was going to end on some lame twist where Darwinia's actually Earth or something. But no, it actually ended very originally, and on an extremely bitter-sweet note.
My only complaint is that for a game called "Darwinia" it's not very Darwinian. I was expecting a lot of heavy symbolism reflecting Charles Darwin's Theories of Evolution, but there's only a few, very subtle, nods to evolution where you can upgrade the troops of Darwinia. In fact the game's sequel "Multiwinia" is more Darwinian, since the Darwinians are battling eachother for supremacy.
There's actually more symbolism for religion than anything else. But don't be alarmed by that, the symbols are very subtle.
I can't recommend this game or the team behind it enough. I've never seen anything like Darwinia, and I'd say you should check it out too.