Kill-a-Watt Energy Meter
Back in 2006 I built up a Dick Smith kitset energy meter. It measures electricity usage for individual appliances.
In late 2008 I bought a more accurate Kill-a-Watt (version 2) meter from Alternative Technology Association for AU$80. The old meter gave readings up to 10% wide of the same appliances with this new meter.
I use this to try to work out what I'm spending my power on - and therefore what options I have for reducing power usage.
They're available in New Zealand now for NZ$69.95 + NZ$9.95 postage.