This seemingly innocuous little device provides protection to your vehicle in a number of ways. Sitting between the battery and electrical system, it can automatically disconnect the battery to save it from draining dead. Additionally, a heavy-duty model allows for an interior remote pushbutton so you can reconnect once you're ready to start. Finally, you can hide a kill switch as an antitheft device. Depending on your application and the type of battery you use, prices range from $70 to $150.