If you use Firefox and do internet shopping, Invisible Hand is a great add-on. It sits there quietly and unnoticed in your add-ons in Firefox, until you start looking at a product on the web that it has found cheaper somewhere else. Up comes a discreet yellow bar at the top of your screen telling you where to go to get it, and any other price offers, as in the screenshot below, where it turns out Asda have a better price on an academic text book than Amazon.