Chrome/Firefox/Safari: Google Shopping offers convenient price comparisons, but as of September 2012, it's been leaving out the biggest retailer of them all: Amazon. Shopping Enhancer is a browser add-on that fixes this problem by automatically inserting Amazon results into the Google Shopping list.
The extension is still rough around the edges. In my quick tests with it, sometimes the Amazon listing wouldn't show up or would show results for a different model than the other retailers are listing (e.g., an older SD card by the same brand rather than the cheaper new model). Also, the extension doesn't show the sales tax, if applicable.
Still, when the extension works, it works as you would expect. You get the Amazon price listed at the top of the results for easy comparison with the other retailers.
Shopping Enhancer | via Everything Finance Blog