Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
PriceCute - Keep up with your competition's pricing on Amazon
PriceSpy - PriceSpy helps you save money when shopping online.
Savelist - Found products you want but not ready to buy?
Pricy - Amazon price watch
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.