FaceRange detects faces from your phone camera and use that information to calculate estimations of distances from you to that person. If the person is too far away and can't be "seen" by the face detector, you can use the zoom slider at the bottom of the screen to zoom in on him/her to get a better result.
Idea Matters is the creator of the app that can be found in the Tools category. Over 0,5K users downloaded and set up FaceRange and its most recent update was on Jan 18, 2013. The overall rating for this application is 3.6 on Google Play.