Wired has an article on garden tech which includes this excellent idea for those (like moi) plagued by deer (and/or whatever) eating their way through the garden.
Unfortunately it appears to be only a prototype from 2012 - it's not clear whether it went further than that, but you can buy the drone and modify it yourself, if you have the skill:
It's a Parrot AR. Drone ($300) linked to infrared motion detectors and a computer-vision nav system on an Arduino microcontroller board. When invaders trip the detectors, the drone executes a defensive pattern, then returns to its base to recharge. DIY instructions at gardengnomedrone.com.