I've previously used a standard kill-a-watt device to check on power usage around my house. I wanted to be able to measure more then my standard three prong, 120V receptacles however.
If you want to measure 240V appliances, I have stumbled across someone who built something to do just that!
If you want to measure things which are hard-wired to your panel, or your whole panel itself, You can look to openenergymonitor.org for information and their shop where you can order any of the parts you would need. I haven't ordered anything from them yet, but they are based in the UK.