PriceCute - Keep up with your competition's pricing on Amazon
RetailMenot - The RetailMeNot mobile app allows you to find deals on the go for both online shopping and in store shopping.
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Coupons.com - Coupons & Coupon Codes, Savings & Deals
Pricy - Amazon price watch
Slickdeals.net - Slickdeals: The Best Deals, Coupons & Discounts on everything