If you've used the iPhone, you'll know about its motion-sensing features, used mainly to flip photos from portrait to landscape viewing or to do the same thing for websites.
However, developers creating web games for the iPhone can also tap into the feature, and an increasing number are. The latest is Under Clouds Games with its Gravity game.
It's a simple puzzle game with you guiding a ball around mazes collecting pegs, but the hook is that you flip the iPhone to do it (although there's also a handy 'gravity kick' button to help).
It's a neat and innovative concept, although like many iPhone web games, its main effect has been to get us thinking of what'll be possible when true native iPhone games are released.
Anyway, if you're reading this on an iPhone, you can play Gravity here (and if you're on a PC, that link works too, so have a look).