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.
A cool tool is anything useful that is superior to comparable items. If you think this tool is inferior suggest a better one. You are welcome to insult a tool, but comments containing insults to individual people will be deleted. Corrections of fact are always welcomed, if stated politely. Recommendations of better tools are dearly wanted and may be elevated to the front page.