Amazon's Price Check, designed to let shoppers compare items on store shelves with their cost at Amazon(AMZN), threw Amazon's retail rivals into a tizzy. An even more powerful weapon is SnapTell, which Amazon acquired in 2009. The app (available for iPhone and Android) has gotten few updates since then, but it still works well.
Take a picture of a book, DVD, CD, or video game you're interested in buying and SnapTell will use image recognition to search for prices both online and at stores near you. The service also offers up ratings and links to Google, Amazon's IMDB, Rotten Tomatoes and other sites so you can read about the item you're looking to purchase.