I don't know about you, but I find it next to impossible to pull the trigger on an online purchase without first comparing prices across a number of sites. This habit is especially evident when I'm shopping for pricey electronics.
NewEgg has been my go-to site for years for electronics and computer equipment. The user reviews on the site are invaluable, and Newegg often has the best price. Even so, I feel the need to price shop before making a purchase on Newegg. With the Hover Hound extension for Chrome or for Firefox, I can get my needed price comparisons without leaving Newegg.
The extension adds green Hover Hound buttons to search results pages and product pages on Newegg. Hover over the button to see prices from Amazon and TigerDirect. Click on the button to open a new tab of the product on either Amazon or TigerDirect -- whichever of the two has the lowest price. From the window that appears when you hover over the button, you can also access a price chart of the product that shows how the price has moved over the past two months from the three retailers.
In Hover Hound's settings, you can show only Amazon products that are Prime-eligible, and you can have it calculate tax if you live somewhere where you must pay tax on Amazon purchases. You can also enable PassMark CPU Benchmark Comparison for certain CPUs to compare their performance to the competing chips.