Collection of PCA sites

Production Differentitation PCA:
http://pricescan.com/ -- often biased to one brand in one particular category

Product Evaluation PCA:
http://www.bizrate.com/

Consumer Preference Identification PCA:
http://www.epinions.com/

Derivative PCA:

www.bestbuyfinder.com
http://dealtime.com/
http://www.shopping.com -- (rebranded from dealtime.com) close-coupled agent design that integrates different PCAs into its architecture -- while you start searching it gives you product suggestions, an advocate agent would give you products related to your preferences since it got to know you over time

An aggregate of the 3 main PCAs with social tagging and customer reviews
http://www.pronto.com/

Refine as you type comparison shopping agents:

http://www.smarter.com/
http://www.become.com/