Shy of a good pair of tweezers, I cannot think of a more useful tool for anyone's geocaching arsenal than the multi-tool. In fact many multi-tools come with tweezers; so there ya go.
Multi-tools allow a geocacher to open geocaches that are stubbern to open, remove a geocace from its hiding spot (pictured) and a great retriever of geocaching logs.
You can go very inexpensive with a basic tool or all the way with dozens of functions. Any way you go, having one of these handy dandy tools on you can be a life saver.