The December 2012 issue of the Australian electronics magazine Silicon Chip features the USB Power Monitor. It uses a PIC18F45K80 to monitor the power consumption of a USB device. The board acts as a bridge between the host and the device with a USB A male on one side, and the USB A female jack on the other.
I hope somebody make an opensource version of it.
Very cool gadget.
SC has done a lot of cool stuff over the years
FriedCircuits offer a similar device, the USB tester. It stands between the USB port and the device being powered and exposes the usb wires so you can check current/voltage with a multimeter. It’s well constructed.
It doesn’t have its own display so far, but FriedCircuits are designing an OLED display as a “backpack” for it.
It’s available on https://www.tindie.com/shops/FriedCircuits/usb-tester/
[No affiliation, just a satisfied customer]
Silicon Chip actually put out a simpler version without a meter a couple of years back, you just use your Multimeter, similar to the Fried Circuits one. but with a small series resistor to measure current without breaking the circuit.
Partial article here http://archive.siliconchip.com.au/cms/A_112479/article.html
PCB pattern PDF for above available here http://www.siliconchip.com.au/Shop/Download/1017/744
PCB available for purchase here http://www.siliconchip.com.au/Shop/8/768
Unfortunately you can only get the full article if you are a subscriber or pay for the article, though it’s very simple. I do have the print copy around somewhere.