Measuring Power Consumption?

rhadc asks: "Like many Slashdotters I have a pet network, spare parts around, and a number of configuration options. Unfortunately, I pay a power bill. I'd like to know how much power I'm using and how to optimize my numbers, but I don't know of any cheap way to find out. Would a 486 laptop be a better firewall than a 486 clone + UPS? Just how long will system X and system y with monitor z last on my my cheap UPS? Does anyone have insight into measuring this stuff?" For this, you can add up wattages on each device, multiply by some arbitrary "fudge rate" for local conditions, and get an approximate rate; or if you are electrically inclined, maybe you'd grab for the nearest ammeter and rig up a way to read what the line is drawing; but is there some device that could simply plug in between your UPS/Computer/Power Strip and the outlet and tell you? If not, how difficult would one be to build?