I take my HTC EVO 4G with me everywhere I go, and lately I've been going to a lot of places I never have been. Google Maps and Yelp have helped me for the most part in unknown towns; they find me a place to eat and they get me there. But Layar adds so many . . . well, layers to that experience.
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.
Want to know if it's safe to walk? Open up the SpotCrime layer and see how many reported crimes were in the area. Want to know if there's any good art sprayed on the walls along the route? Pop in the streetArt layer. Want to see all the Wiki entries pertaining to your walk, or like the neighborhood so much you'd like to live there? There're layers for that as well.
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.