priceprotectr

"Use Price Protectr to track prices of things you're thinking about buying, and save money even after you buy.
There are lots of stores out there that offer price protection policies -- when the price drops on an item you've purchased, they'll refund you the difference. But there's a catch... it's up to you to watch prices.
Price Protectr makes it simple to watch prices, keep track of your purchases, and get rebates off price drops."

Go here to see a video of how it works: http://vimeo.com/2350805
and here for the website: http://www.priceprotectr.com/