From Carlo Zottmann:
An extension for Firefox to help you shop better! BetterShopper can convert all displayed Amazon.com/.co.uk/.de/.ca/.fr/.co.jp prices to your favourite currency: US$, CDN$, EUR, GBP or JPY. And it'll do so using daily updated currency exchange rates.
Also BetterShopper will lookup the prices of the currently displayed Amazon product in over a dozen stores across the web, and then it'll display those in a neat little box right there on your current page. All that without the need for you to leave Amazon.