Tape

You don't really think of them as "tools", but duct tape, caution tape and safety tape are often as important as any other tool in your tool box. For instance, duct tape was the key component that allowed the Apollo 13 astronauts to retro-fit the crippled command module's air filters to the air cleaner in the lunar module so it could be used as a temporary "lifeboat". And you can easily understand how caution tape (or safety tape) is very important for keeping onlookers away from work zones. If duct tape is critical enough to hitch a ride on every NASA space flight, it certainly deserves a place in your tool box!