Invisible Hand is a Firefox extension that observes products you are looking at online,and unobstrusively,recommends another site that is selling the same product for a lower price. It will tell you how much you would be saving by shopping at the cheapest option it could find,and offers other online retailers that are selling the same product (in case you know.. you don’t think Best Buy is just awesome…). This is a really useful extension,that has minimal impact on your browsing experience,and can help save you money on those impulse buys,or times when you didn’t realize that you could have saved some money by just going to Amazon instead. (Found on Lifehacker)