BidJig

BidJig is a free browser extension that answers one question on any product page: is this cheaper somewhere else?

It reads the page, checks 430 partner stores and marketplaces for the same product, and shows what it costs elsewhere with shipping to your country included, so you compare the real total rather than the sticker price. It tries coupon codes at checkout and pays back a share of what a sale earns.

If it cannot find the same product, it says nothing rather than guessing. Free for Chrome and Firefox.