InvisibleHand: Finds Better Deals
InvisibleHand is a Firefox extension which can notify if the product you’re currently browsing available somewhere else at a cheaper price. It searches competing merchant sites for the same product and when available displays a discreet notification showing a better deal.

Apart from showing the best price among retailers, it also displays the percentage and actual money saved. The application currently supports more than 50 of US and UK retailers including Wal-Mart, Dixons, Amazon, Best Buy, Newegg, PC World etc.,
Features:
- Firefox extension for real-time price comparison in your browser.
- Find the cheapest offer for any product.
- Displays the best price and actual money saved.
- Currently supports over 50 US and UK retailers.
Check out InivisibleHand @ www.addons.mozilla.org/en-US/firefox/addon/11377 (by Srikanth AD from Techonaut )



That sounds like a great add-on I am going to check it out, if it can givr good resultes and save money it will be a hit.
I use SimilarWeb add-on to find similar sites, I found some sites that sold what I was looking for just cheaper. this add-on can replace it for that usage.
http://www.similarweb.com