Whether you are shopping online for books, software, gadgets or anything else, you probably know that comparison shopping engines can save you a lot of money. Just type the name of an item that you are looking to buy online and these sites will instantly compare the price of that item across all the different online stores so you always get the best deal.

See: Download Free Amazon Books

Compare Prices of Items in Real-Time

If you shop online quite frequently, you should try Invisible Hand — an excellent extension for Firefox and Chrome that will not only help you find the best deal on the Internet for any product but will even save you a trip to the price comparison site.

It works like this. Let’s say you are on Amazon.com to buy a Wi-Fi router. Invisible Hand, working in the background, will automatically recognize that you are on a shopping website and it will immediately start fetching prices of that same item from all the other online stores.

If that item is available for less on another shopping website, that information will be highlighted prominently in a yellow bar so you are unlikely to miss the deal. The prices from all other vendors are displayed in a neat drop-down so you know where to go incase that product is out of stock on the current site (which, in this case, is amazon.com).

And other than popular shopping websites, Invisible Hand can also display price notifications directly in Google if your search query matches a product name (like “buy iPod Touch”).

Are you getting the best deal?

Invisible Hand works in real-time without you having to do anything. This may also help in those “rare” situations when you are about to complete a purchase and suddenly there’s a price drop on another shopping website. If you have the add-on, it will automatically catch that fluctuation.

Related: Which Amazon Site Should You Use for Buying Books

Watch this quick screencast to know how Invisible hand works in your browser:

The algorithm only takes the item price into consideration so you’ll have to manually compare the shipping and handling charges to find the best “overall” deal.

Invisible Hand work with all popular online retailers in US and UK including Amazon, Wal-Mart, Buy.com and even eCampus (for college textbooks).