A Hammer..

As the story goes, a Russian Cosmonaut was in the middle of performing some routine maintenance on the outside of the International Space Station when he found himself without a crucial tool needed to complete one of the tasks at hand: a hammer. Rather than abort the spacewalk and leave the task uncompleted, the resourceful ISS crewman removed his Fortis Official Cosmonauts Chronograph strapped to the exterior of his spacesuit, and used it as a blunt instrument to hammer a rivet back in place and complete the repair.