Just in time for ghost-hunting season (because what else would you use it for?), Seek Thermal is a neat sensor for your smartphone. Most objects give off heat and thermal cameras like this capture that radiation even in the absence of visible light. The Seek Thermal extension (which plugs into the USB adaptor on Androids or iPhones) actually has a lot more uses than investigating haunted hotels or landmarks. Although slightly less exciting, you can use it around the home to find studs, clogged drains, and water damaged areas, which makes it pretty handy around the home. If you're a camper, you can also scan a dark landscape to search for bears, the boogeyman, or el chupacabra.