Update: Maplin have discontinued the electricity monitor used here.
Software for monitoring the electricity consumption of a home with a Raspberry Pi
Full write up: unop.uk/dev/raspberry-pi-electricity-monitor
Copy the www folder to /var for httpd to host and the python file to anywhere.
Install pyserial with apt and plug in the receiver.
To run execute sudo python raspberry-pi-electricity-monitor.py & or add this to /etc/rc.local to run at boot.
Visit your pi in a browser to see the data.
I've added a new script (maplin2emoncms.py) which you can use to push the data to OpenEnergyMonitor emoncms running on the Pi.
Thanks to dygraphs for an awesome library and Raspberry Pi for the great hardware.
MIT licensed
See also this alternative.