Huge-River - www.huge-river.com

A simple site that plots the price history of items sell so you can see if you are getting purchasing the item for the best price. What I did not realize until I had found the site was that amazon.co.uk's prices go up and down so often (apparently as often as every hour).

The was an added feature I liked that it also pointed out prices in different countries across Europe which for me was of interest but If you could speak the language I imagine would be useful for saving you additional money.