A unique and useful Firefox plug-in has launched into beta in the UK that could save you money on your online shopping without the pain and frustration of having to visit price comparison sites.

"InvisibleHand" lets you know instantly if you can find a cheaper product elsewhere as you are browsing and provides you a link to hop over to the other shop and buy the item.

The example given is say you’re buying a DVD from Amazon, InvisibleHand will pop up and "discreetly" tell you just beneath your address bar if you can buy it for less elsewhere.

The plug-in looks at Amazon, Play.com, Argos, Comet, John Lewis, HMV and PC World, among others and claims to still search for deals even if you are browsing a product it has not "seen" before.

The plug-in is free to download and use and claims to have found over £15,000 of savings so far with the average saving at £8.