Darkspot shows how dark the night sky is at any place in the contiguous US on the Bortle scale (1 to 9) with an SQM reading, tells you whether tonight is worth going out (moon, twilight and cloud cover), and finds darker skies nearby with drive times. Save places, get optional great-night alerts, and switch to a red night-vision mode at the dark site.
Darkspot is an iPhone app. It is not on the App Store yet.