I do a lot of work on my own cars, mainly because I’m cheap and I enjoy the work. I own a couple of test lights to verify that circuits are working, and I keep a small array of test wires that serve as jumpers, but the Power Probe III jumped out at me because it performs those functions plus many more.
Just hook the unit up to the battery or the cigarette lighter/power point and it’ll serve as a switchable (positive/negative) lead for testing circuits and relays and checking for continuity. It works as a volt meter and can detect short circuits. It also sports a flashlight, handy for locating a wire or reading that fuse panel under the dash.
The Power Probe III Kit comes with a 20′ cable and sells for $100.