My favorite golf gadget/accessory is my point and shoot Leupold Range Finder. I use it diligently through out every round and teach students how to use it effectively as well. Obviously, the range finder helps me and my students find accurate distances to various targets. However, most amateurs only use them to find the yardages to the flagstick. Realistically, they should be using them for tee shots, layups, and approach shots.
When using the range finder off of the tee, golfers can determine how far the hazards, bunkers, and other trouble areas are. This can help golfers make the correct club selection for the tee shot. As for lay ups, golfers can both see how far trouble areas are and determine how far they want their approach shot to the green to be. And with approach shots to the green, golfers can figure out the distance to carry the hazards and bunkers to ensure they have enough club to land on the green.