Invisible Hand is a unobtrusive plug-in for Firefox, which subtly alerts you whenever a deal better than the one you are viewing is available.
Say you are looking to buy an LG LCD TV. You go to Best Buy and decide upon a certain model. And while you are reading the page, a small toolbar drops down to inform you that the same product is available at Buy.com for $200 less – a saving of 17%. It then provides the link to go to Buy.com’s page for the product. Additional links are provided that lets you look at the same LCD TV model at other shopping sites.
Invisible Hand stays out of the way most of the time and shows up only when there is a lower price available on another retailer. However, it doesn’t take into account shipping and handling charges. The plug-in works across all major retailers like Amazon, Buy, Best Buy, New Egg, and about 50 others.