Does anyone know of any good software that will measure my PC's power consumption while I mine ethereum? I'm aware I could buy something to measure it on the socket, but free software is well, free.
Computer hardware currently does not have the inbuilt instrumentation capabilities to measure power consumption across the whole system.
http://www.pcgamer.com/how-to-measure-your-pcs-total-power-consumtion/ talks about software that may be able to calculate some estimates, but these requires calibration, and calibration would require a power meter at the wall.
Your power supply is not 100% efficient in converting energy (https://en.m.wikipedia.org/wiki/80_Plus). Software to measure power usage would have to measure the power consumption of the input to your power supply as this represents the energy cost for you.
There are power supplies that measure their power consumption. Example are http://www.highpowersupply.com/product-ps-hpc480102df.html and https://www.thermaltake.com.au/products-model.aspx?id=C_00002723 . However, you won't easily find these in your computer stores. They will have other restrictions (e.g. not high enough wattage to supply your 1+ GPUs).
It is cheaper and easier to purchase a wall socket power meter.