I recently got my hands on some small CT sensors. They are used to measure the current in an AC circuit. This is great if you want to see how much current is being used in your home or office. With this information you can the calculate the running cost of your circuits.
E.g. you will usually have separate circuits for different electrical items in your home or office. So you could measure all of the power points or all of the lighting. This will help to narrow down the costs.
In commercial/industrial situations you could use it to measure large fridge circuits, motor circuits etc.