IsThereAnyDeal Everywhere adds a quick price overview for each link to Steam store it can find on a page.
A small icon will appear next to each Steam store link. Moving your cursor over it will show you a tooltip that contains current best price and historical lowest price from authorized stores, as tracked by IsThereAnyDeal, as well as some quick and useful links to IsThereAnyDeal and SteamPeek.
Try it on Steam, SteamGifts, Barter.vg or reddit ;)
The original author of this extension is Zoltan Wacha.