Invisible Hand – Automatically Gets You the Lowest Price On Shopping and Flights
http://www.getinvisiblehand.com/

It’s a browser extension which discreetly notifies you if the product or flights you’re shopping for is available more cheaply from another retailer or travel site. The notification provides a convenient button which links you straight through to the relevant page on the competing website. It also provides a drop-down menu, so that you can see the prices at the other sites which sell that product or flight. This will be added to ShoppingBots and Online Shopping 2011.