Shoptagr - Personalized sale alerts.
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Keep Shopping - Shopping assistant to help save items from the web.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.
TrackIf - Alerts you if the price of your favorite products drop
PriceSpy - PriceSpy helps you save money when shopping online.