I got a batch of cables that had a few bad wires in them, so I needed to test every cable. Checking all the wires with a multimeter was out, so I built this simple cable tester out of a proto board, some headers, an LED, a resistor, and some wire. It just completes a circuit through all the wires in a cable; if any wire is bad, the LED will not light. Simple concept, but it allowed me to test about 300 cables in about half an hour. A video of a few cable tests is below: