Say you want to buy a digital camera and pick out a particular model on Amazon, for example. If the same product is available for less at buy.com (or any of the 220 retailers currently supported in the US, UK and Germany) InvisibleHand will show a discrete notification below the address bar with the price and link to the product page at the competing retailer. An option to view all other results found is also offered. According to the developer, InvisibleHand even knows the price of products when the retailer tries to hide it using add to cart mechanisms and the like.
Following a recent update the add-on also gained the ability to display the cheapest or most convenient flight when you search for flights on a particular date and route using sites like Priceline, Expedia, or individual airlines.
It's really a time and money saver. The name of the extension is explained on the extension's FAQ page, and I paraphrase: The invisible hand was originally mentioned by the 18th century economist Adam Smith, who argued that in a free market, individuals will pursue their own self-interest, and the consequences of this will be beneficial for the rest of society. Smith didn't have the Internet to help make his point, but we hope he would approve if he were alive today.