Microsoft has acquired Jellyfish, a comparison shopping site that launched in June with a pay-per-action ad model.

Jellyfish acts like a reverse auction through which retailers are ranked according to the amount they agree to lower their prices. The more they drop the price, the higher up in the search results advertisers appear.

On the Live Search blog, a Microsoft rep said, "We think the technology has some interesting potential applications as we continue to invest heavily in shopping and commerce as a key component of Live Search."