I like one small and simple enough to carry in my pocket without noticing it or losing it. The smallest one can get lost easily. But for a tool box, you can get a bigger, fatter one with some gizmos on it that you really think you'll use. What's the most probable thing you need when you don't have a full blown toolkit? First, a good sharp knife blade. Maybe a can opener? A tooth pick? A finger nail cutter? A hair trimmer? An LED flashlight? etc etc So you probably need at least two of them, one small and simple for carrying around and one with more gizmos for a purse, bag or glove compartment. A competing tool is the "Leatherman"
http://www.google.com/search?hl=en&q=leatherman&sourceid=navclient-ff&rlz=1B3MOZA_enUS334US334&ie=UTF-8&aq=t&oq=leatherma