http://www.nextopia.com/
This one is newer, but it looks pretty good. They have the "Narrowing" options like many of the top sites use to help find products based on criteria that the shopper chooses.
http://www.sli-systems.com/
SLI offers a "Learning" search. As people search and choose a product, it learns trends and starts to tailor the results to those trends.
Personally, I went with a Google Mini.
http://mini.google.com/
They're all decent, and the first two have free trials available. The Google one is hardware, so you can host it at a datacenter or over cable / dsl. You can totally customize it pretty easily, too.
If you want to see some sites that use each, let me know. They list a few on their own sites, but I know some of the guys at SLI from attending conferences and know of another customer using the nextopia that isn't listed. Last I heard, google uses our tool site as part of the mini demonstration.
Brian.