Marco Arment's Instapaper iOS app has long been able to switch into a light-text-on-dark-background mode for easier nighttime reading. That's useful, but not so interesting. Other apps, such as Amazon's Kindle app for iOS, do the same thing.
Except that in the just-released version 2.3, he added the ability for the app to switch into dark mode automatically based on whether the sun has gone down. And his method for determining whether it's dark outside is pure genius.