Firefox: Invisible Hand is an apt name for a deal-finding Firefox add-on. Visit a priced item page at more than 50 U.S. or UK retailer sites, and a subtle drop-down bar informs whether you've got the best price out there.
The drop-down tells you the price difference between the page you're on and the lowest price it could find, and offers a link to that retailer and other sites' prices in right-hand buttons. That's all it does, and it doesn't seem to slow down page rendering to do so. Price isn't everything online, of course, and the add-on doesn't factor in average shipping costs, but serious shoppers looking globally won't mind checking out a competitor if a real savings is in play.
Invisible Hand is a free download, works wherever Firefox does.