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]