A product design I saw on inhabitat (a great architecture and design blog that focuses on green and pre fab) is a meter for faucets to see how much water is being used. The theory goes that if are reminded as to how much you are using you will use less. It seems to just be a design/prototype right now but I think it would be great if it becomes a real product.
http://www.inhabitat.com/2007/05/08/isave-faucets-monitor-water-use/