Surprise! The popular iPhone app Yelp has a secret feature that overlays nearby points of interest on live video of your surroundings. This augmented reality feature is available in Yelp version 3.0.0 on iPhone 3GS.
To turn on the feature, called Monocle, make sure "Location Services" are ON. Launch Yelp and select the "Nearby" screen from the bottom menu. Now gently twist or shake the iPhone a few times until the dialog box stating "The Monocle is activated" appears.