Groupon - Each day Groupon offers a deal through local vendors in your city.
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
RetailMenot - The RetailMeNot mobile app allows you to find deals on the go for both online shopping and in store shopping.
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Offers.com - Offers.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.