A box clicker for dog training (or horses, cats, chickens or event dolphins). Press the metal strip lightly and they make a clicking noise. This is a distinct and consistent signal to mark a desired behaviour from your dog. You quickly follow that signal with a motivating reward (piece of food or a toy). Because animals understand precisely which action earned the click and their reward, they learn new behaviours quickly, easily, and enthusiastically. We use clickers in our truffle dog training lessons and workshops and think they are very beneficial training aid.
See the below links for lots more on clicker training: