GoTenna
Anyone who's ever had their power go out or seen their cellular service disappear at a sporting event knows how convenient it can be to have a backup way to contact friends in your general vicinity. GoTenna provides exactly that. These backup communication devices pair with your phone via Bluetooth LE, then use low-frequency radio waves to communicate between each other, offering an easy way to share your location or send out a text. There's also end-to-end encryption and self-destructing messages, so your security concerns should be few, and the GoTennas themselves are water-resistant and dust-tight, so you don't have to worry if you want to take advantage of them on your next outdoor adventure.