3. DIGITAL CAMERA
A picture is worth 1,000 words, and in the case of mechanics it's worth 1,000 words of instruction. Use the camera to document things as you disassemble them so you can reference the photos if you need to—this is especially helpful for wiring, hose, and cable routing. Your smartphone works for this purpose as well, and since photos these days are all digital, you can snap away with abandon. You never know when a photo will reveal that little detail you need to finish your project!