Electrical Power sensor
Current transformer is evidently what you're looking for. If you can disconnect and reconnect the wire to thread it through the transformer, it's easier/cheaper - if you need to attach without disconnecting the wire, you need a split core type as @Tyler notes. 

There's a tradeoff (to the point of a dichotomy, IMHO) between making an accurate measurement and trying to power something from a current transformer. Likewise, if there's no current being drawn, there's no power for your circuit.