Yelp's has cunningly added augmented reality functionality as a hidden feature in its existing iPhone app, for iPhone 3Gs users.
The Easter egg can be unearthed by simply shaking the app a few times (actually, to the amusement of my colleagues, I shook my phone for about five minutes before it decided to work). It's only available for the newer version of the iPhone because it needs to use the compass.
Once activated the AR function works via a new button on the top right of the screen, labelled 'Monocle'. Click it to access a camera plus overlay that features restaurants, bars and other markers. You rotate your phone to reveal the markers on the screen, to help you navigate towards them.
Although this is a first stab at AR for Yelp the app works well. A screenshot shows a marker for Yo Sushi right next to my local Yo Sushi branch.
You can also check out the video below to see how it works:
The Yelp app is available for download from the App Store for free.