I love things that save me money when shopping online. While PriceGrabber and Google Shopping are my two go-to sites for finding the lowest prices on electronics, tons of sites track prices and give the heads-up when something goes on sale or gets its price reduced.
There's also PriceAdvance, a browser plug-in that will pull up competing prices from other online sellers automatically whenever you hit a page with a product and price on it. Its latest iteration for Firefox and IE doubles the number of prices it shows you compared with the previous version.
Right now it only works on 16 major online retailers including heavyweights like Amazon.com, Buy.com, and Wal-Mart. For non-supported sites I'd recommend going with a service like Retrevo (review), which pulls in the prices throughout the day and includes a listing of product manuals--just in case you don't feel like hunting around for the spec sheet.
I've embedded a demo of it in action below.