Stateless Systems, the creators of BugMeNot, PDFMeNot, and RetailMeNot have a new site called Beat My Price that helps you find the lowest price for online goods. You simply plug in the link to something you want to buy, and it will compare that to prices it, and other users have found.
All the while, the system takes advantage of companion site RetailMeNot. This lets you see if you can save even more with the use of online coupons. These coupons don't change a product's total price listing, but you can quickly eyeball ones that might save you a few more dollars here and there.
One nice touch, and something that separates it from similar search offerings is the capability to remove entire retailers from the results--something which can be saved for future searches. In my case, this completely weeded out some of the bogus results I was getting for shady eBay auction listings. Going forward it would be nice to have an option to flag some results to have them removed entirely--at least for the sake of other users.