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A U.S.-centric application (no clues in the name, of course), GasBuddy is an online community of drivers who share and search for gas prices in the hunt for the best deal. The free app allows you to save some cash by searching cities and postal codes for the best deals to fill up the motor.
Developer: Lifelike Apps
Craigslist, the U.S. equivalent of the U.K.'s Gumtree, is a platform for classified advertisements. The Craiglist app is set up nicely -- a newspaper design and simple search functions help you in your search to find items second-hand, rather than splashing out for new products that you need.