While pretty much all smartphones come with a built-in flashlight app, it's nice to be able to get more functionality out of it, which is why a third-party flashlight app isn't a bad idea at all.
iHandy makes a great flashlight app that works on both iPhone and Android devices, and it comes with some added features that the built-in flashlight functionality doesn't have, including a strobe light option that lets you send out a visual alert to anyone that might need it -- great for when you may be trapped in debris after a tornado.
The app also comes with a built-in compass and a mini map if you ever need to find your way home or to a specific location.
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