If you're a photographer—or anyone who spends much time outdoors—and you have an iPhone this is a must have app. Photography is all about light, and light is all about where we are in relation to the sun, so if you're not paying attention to things like civil twilight, dawn, dusk, solar transit (high noon), you're missing out.
This app takes your GPS location and immediately gives you that info for the current day. You can also select any date/location you choose.
One of the best apps I have for the iPhone.