Floating islands, and what happens when you build on them.
More screenshots at flickr
Cities in the Sky is a game about building on a floating island whilst keeping it balanced, and keeping the little people living on it alive.
Buildings are lowered from the sky for you to manoeuvre into position, in a Tetris-like fashion. The island tilts in reaction to buildings being placed on it, and as the island tilts, the buildings and people are more likely to fall off.
To get a good score, you need to keep the people happy by building in a particular fashion. Of course, this fashion is opposed to keeping the island physically balanced, and therein lies the fun!
You can read about the development of Cities in the Sky on my blog; it tends to be quite technical (and sporadic), but maybe that's your thing.
Cities in the Sky gameplay as of March 2011
More videos at YouTube