I, like many other people, have pets. I want to train them to do things, and the best way to do this is to offer them a treat when they do something that pleases me. Unfortunately, you have to offer the treat immediately, or the pet will get confused as to what it's for. This has meant for me carrying around a bag of treats. I imagine a better option.
It would be a small, gumball-machine-ish device, which would connect via radio to a small clicker. I would set down the gumball-machine, and keep the clicker in pocket or hand. When the pet pleases me, I click the device. This both makes a clicking sound and activates the machine to drop a treat for the pet to consume. The clicking also makes the pet associate the click with "You did a good job -- you'll be getting a treat very soon."
Maintenance wise, I will need a way to refill the treat-supply, and it is important to avoid jamming the device. I'm sure gumball and toy vendors will have a legion of advice on that topic. Also, low battery (or other power sources) warnings so that I know to change them. I want the click to be loud enough so that my pet can obviously hear it, but not so loud that it bothers people around me.
Training would go faster if there was also a "no, that was bad" indicator, but I don't know any universal enough. I expect this to work with dogs, cats, parrots, ferrets, and lizards, provided that the animal can hear, is capable of living outside a cage (the way that a dog can and a chimpanzee cannot), and is motivated by food. (I don't anticipate this would work well for, say, a panda.)