I got a tip about InvisibleHand, a new Firefox plug-in for price comparison shopping that’s been getting great reviews on some of the top tech blogs.

We all know that sick feeling you get when you find something at a lower price than what you’ve JUST paid. There are zillions of price comparison engines out there, including Shopzilla and PriceGrabber, but this is handy because it does more of the work for you.

In short, InvisibleHand shows a discreet notification when the product you’re browsing can be bought for a lower price elsewhere and provides a link directly to the relevant product page at the competing retailer. It is currently supporting 30 retailers.

Features include:

• It stays out of your way – InvisibleHand remains invisible until it finds a deal on a product you’re shopping for.
• Real-time prices - Unlike price comparison sites, InvisibleHand refreshes its prices every single time you use it.
• It learns as you use it – InvisibleHand will search for deals, even when you browse a product it hasn’t seen before.

At the time I’m writing this, InvisibleHand’s tracker claims to have found £3,539,595 worth of savings, an average of £8 per user.

Sound good? Download here.