The classic GI Can Opener is an excellent tool. Small enough for a key ring, or worn on a lanyard around the neck, it has been of assistance in repairing a fuel pump, tightening screws, and yes, opening cans. All for ~99 cents. It is sold at many locations online, as well as most Army/Navy stores. In years past I have given them as gifts. Invariably I have heard similar stories from friends, of truing pins on cell phones, adjusting fishing gear, stripping wire, etc. The URL below gives its extensive history.