The Retrievor is an eco-friendly GPS tracker that is no bigger than a U.S. quarter. The device is powered by the sun (it can also be charged using a micro USB cable,) so you don't have to worry about its batteries dying. It works using an Android or iOS app that lets you set boundaries and receive a notification via text or alarm when the Retrievor has crossed the pre-set threshold.
In addition to being able to set boundaries, the Retrievor also lets you find the beacon's location and see how its going and the route it took to. The Retrievor is currently seeking $80,000 worth of funding on Indiegogo, and a pledge of $179 is enough to secure one of your own.