Sonar Ruler is an iPhone app that uses clicking sounds to measure distances.
Basically it uses the iPhone's speaker to send out a short click sound and it then measures how long it takes for that sound to bounce off of something in the environment. It can be quite accurate to within an inch or so when used in the right situations. I'll say right upfront it's not perfect, and cannot measure something small like a person. It works best on a large flat surface that is perpendicular to the iphone (like a large wall.)
Things that come to mind: "reverify our range to target...one ping only", the boy who sees by clicking, Daredevil, the last 30 minutes of The Dark Knight, and this app is going to be many architects' new best friend. (thx, matt)