City builders, by their very nature, have you exploiting the landscape in order to make cities. Terra Nil—Reclaim the Wasteland, a free game created for Ludum Dare, is pretty much the exact opposite. Rather than constructing cities, you're recreating the natural world, with the aid of some advanced technology.
Terra Nil begins in a desolate wasteland, a procedurally assembled patch of land that you're attempting to convert into a lush field of green. The game is fairly relaxed in how you go about this. There's no time pressure, and—depending on what difficulty you play on—plenty of resources with which to place structures.
There's not a huge amount to it (it is a jam game after all), but I love the concept, and the aesthetic of this reverse-city-builder. It's particularly satisfying to be given a patch of ruined earth, and to rejuvenate it to an area of verdant green.
For more great free experiences, check out our roundup of the best free PC games.