For everyone who's interested in building an open hardware power meter, I thought I'd put up a little page with some of the resources I've come across so far:

Green Soda Project

It's a project (now finished I think) to deploy plug level metering within a student hall of residence. Very interesting!

Green Soda Hardware (Eagle Cad format, BSD licence)

All the PCB design for the above.

Analogue Devices Power Meter IC's This one seems to be the best candidate from their range for a plug/circuit level meter. 

To add a few others to the list:

Enerjar - these guys won a greener gadgets award. They decided to stop development and support on their project due to not having mains isolation. Their website seems down though at the moment. When its back up it does have a wealth of information.

PowerBox Cliff Jao and Xi Guo from Cornell University. A really great page and good safe electronics.

Tweet a Watt by ladyada. A kill-a-watt meter hack. It has wireless Xbee communication, connects up to Google Visualizations and Twitter. Loads of great info to learn and build from

Jason Winters diy smart meter using the sensors connected up to an ioBridge.com and then google charts.

MPGuino a car hack to get mpg. Interesting potential for merging with data from home energy use.

OpenEnergyMonitor

The website for the energy monitor I bought along with me too the last homecamp. It uses an Arduino and Java and is fully open source.

Flukso - community metering

An open source hard- and software community metering application, currently in beta. The software + hardware schematics and layout reside in the Jokamajo repository.