Mr. Comparison Shopping himself, Brian Smith, posts that IAC/InterActive (the company that owns Ask Jeeves, Citysearch, Ticketmaster, etc.) has just released a comparison shopping service Pronto. Look for a review Brian's site shortly.
We will also plan to give it a go. Btw, I used the word service instead of site because Pronto requires the user to download a 2MB app. At the moment, it's IE only available for Windows 2000 or higher. You can learn more about Pronto here and the service itself via these FAQ's.