A washer made from a split, twisted ring. It doesn't guarantee that a fastener won't vibrate loose, so if things keep loosening, check the joint and look into a locking method that suits it.
A black bugle-head screw for fastening drywall to wood or light steel studs. If the board is lifting, also check the framing and the board itself for damage.
A metal band that tightens a hose onto a pipe or fitting so it can't slip off. If it leaks, also check for cracks in the hose and damage to the fitting.
A U-shaped clip that clamps the end of a wire rope folded back into a loop. If one has come loose, also check the wire for damage and how it was installed.
A U-shaped connector with a removable pin for joining ropes and rings. If it is rusted, don't just replace the pin; inspect the body too and use a combination approved by the manufacturer.
Pliers that lock shut on whatever they grip. They are used on stripped screws whose heads stick out far enough to grab; a countersunk head sitting flush is hard to hold.
A tool with toothed leaves that you hold against a thread to find its pitch (the spacing between threads). To know whether a nut fits, you also need the diameter and thread series.