InvisibleHand is a Firefox Add-on that will alert you if a product you’re viewing can be purchased on another Website at a lower price.

The way it works is simple:  Browse for products on sites like Best Buy, Amazon, OverStock, Borders, etc. and if a lower price is available somewhere else, InvisibleHand will slide open (almost unnoticeably) and show you how much you can save and give you a link to the product page.  It works for many items like electronics, beauty products, books, toys, and video games.  A full list of supported Websites and products can be found here.

I forgot that I had installed InvisibleHand yesterday afternoon – I was planning on taking it for a spin but got sidetracked.  This morning, when I was briefly browsing OverStock for a new comforter, I was pleasantly surprised to see that the item I was thinking of purchasing was $10.00 cheaper on Amazon.  That’s how invisible InvisibleHand is.  I completely forgot it was installed until there was a good reason for it to make an appearance.

To see InvisibleHand in action, watch the screencast below (best viewed in HD in full screen):