The Shopping Helper has many features to help you shop, though everything really revolves around keeping Shopping Lists. Some people may choose to use their product lists simply for its bookmarking convenience, while others will appreciate the utility of tracking prices across their Favorite Stores. Pronto's Shopping Helper can also automatically track price changes, providing both Product Alerts and Prices Histories. This works for products on any web page, but usually works better when products are added from Pronto.com pages where there is more known information about the product. This version only supports shopping sites that use U.S. dollars.
A demonstration video can be seen here: http://www.youtube.com/watch?v=EXBgdo6ktmw&fmt=18