The Sunrise Sunset Calc is a free application that lets you find the following timings for either your current location or any given location on the earth:

1) Sunrise and sunset
2) Civil Dawn and Dusk
3) Nautical Dawn and Dusk
4) Astronomical Dawn and Dusk

The computation is done using the formulae at http://williams.best.vwh.net/sunrise_sunset_algorithm.htm. Note that since these formulae do not take into account the elevation of the place, the computed values may be slightly inaccurate for places much above sea level.

All of the features currently supported by the app are free.