A measuring or rafter square is a great addition to any tool box. You can use it to mark a board for square or angled cuts. It fits securely into a tool belt and can be used to check the trueness of an angle or as a cutting guide for a circular saw. It's also handy when you're working with large-dimensional lumber or when you're laying out rafters.
In many houses, the electrical system is grounded to the water service entrance pipe. Since the entire plumbing system is connected to that pipe, be careful not to come into contact with plumbing pipes when you’re working on electrical repair tasks.