My newest project using Arduino. to monitor and document my electricity consumption.
I still have to clean my code, but your feedback suggestions and comments are always appreciated!
thats a nice setup! with the google api you can make some nice charts without effort from your data 
I've had a system running for quite some time, It does Gas and various temperatures and controls the central heating system too. My meter is in a dark cupboard indoors, but its a spinning disk job so I have a 'reflective object sensor' (Phototransistor and IR LED in one package) counting how long it takes for the black mark to come round. The gas meter has a built in magnet on one of its reels for attaching a sensor, so it was just a matter of taping a reed switch in the right place.
A small linux server which also acts as a firewall and file repository keeps track of the 'score' from the arduino using Bash scripting.
It also uploads its data to Pachube, although I don't really use the online facilities of Pachube.
Graphs here : http://pluggy.is-a-geek.com/index.html
Pachube : http://www.pachube.com/feeds/2480
Somewhat out of date writeup : http://pluggy.is-a-geek.com/arduino/index.html