Colt 1911
Swiss army knives / multi tools
I was fiddling with a P-38 can opener today, trying to figure out how to keep it from opening up in my pocket. I dawned on me that a safety pin would do the trick. I'm probably not the first person to do this, but it sure works. And the safety pin has its own utility. Other folks have used a paper clip, shrink wrap or a rubberband, but I like this one the best.