Kill A Watt is a neat little device that you can plug into your wall, then plug other devices into it, and it will give you continuous readings of how much energy it is using, line voltage, power factor, and total power consumed. I had never bothered to buy one, because my dad and I put together a similar device well before this was available, using just a gang box, some banana terminals, an amp meter, and the ability to multiply by 120 in our heads.
As luck would have it though, one of my friends was kind enough to gift me a dead Kill A Watt that died while he was testing exactly what they mean when they say "do not use for loads greater than 15A." I was optimistic that this repair would involve little more than replacing a fuse.
An entertaining point to notice is that the load resistor, which the Kill A Watt measures the voltage across to figure current, isn't really a resistor at all, but simply a piece of heavy gauge copper wire, marked R2.
Amazon is currently selling them for the surprisingly low $17.
I am not so much a fan of how the Kill A Watt manages to cover five outlets when you plug it in, but I guess most people actually plug devices into those things mounted in the walls, and not exclusively power strips like I do (my 15 outlet data center power strip helps, but I still have three more normal-length strips chained off of it).