Wikipedia
Jellyfish.com
Jellyfish.com was a reverse auction online shopping site website. The Middleton, Wisconsin-based company was acquired by Microsoft in 2007. On May 22, 2008, Microsoft officially announced the cash back service as part of their Live Search group of tools. The site was shut down at midnight, February 16, 2009.
It is the first website exclusively using "cost per action" for shopping, and the first to give ad revenues to customers by means of rebates on purchases.