After it was pulled in December due to some bugs, Layar is back in the App Store, available for iPhone 3GS users.
Just in case you need a run-through, Layar—which is also available on Android—is an augmented reality app which lets you discover people and places around you, based on different layers you select. It's only available for the 3GS as it needs a compass to work, and also uses the camera function to display your surroundings on a cool grid. Definitely worth downloading if you haven't already. [Layar via Mobile-Ent]