Layar is an augmented reality application that makes use of your camera, your compass, and your GPS to overlay information - both graphically and with text on your screen.
Want to find restaurants and reviews that-a-way? Point the camera that way, put the layer to Yelp, restaurants, or McDonald's Finder if you're into that, and point away. A heads up sonar circle will tell you where things are located around you and pointing the camera at them will give distance, reviews, pictures, etc.
Twitter integration means you can see who tweeted in the area assuming they included location, along with their picture and tweet. There are also layers for museums, Starbucks, Groupon, bars/pubs, apartments, camping, careers, cafes, nudism, webcams, YouTube, and tons more.
Layar itself is free in the market. Some of the layers do cost some money, but the majority do not. Layar can also kill your battery pretty quickly, so you may want to carry a spare with you if you plan on some urban adventure.
Links: Market (EVO) | Market (web)