Results are now powered by the Alexa Web Search Platform (Beta).

From their blog:

<< While sample applications were useful demos of what's possible, some of our customers have asked, "Is it possible to build a real search engine using the platform?" It's a good question. So we gave it a shot and here's what we came up with. >>

http://awis.blogspot.com/2006/09/released-today-alexa-search-beta.html

Some sample "vertical" search engines that also use this platform (taken from their post):

Camera Image Search: http://www.alexa.com/site/devcorner/cis
Robots.txt Search: http://www.alexa.com/site/devcorner/rdt
ZIP Search: http://www.alexa.com/site/devcorner/zip

I wonder if they'll migrate http://a9.com to use this platform too...?

First A9 and now Alexa. Ay! :(

They've used MSN Search for some time.

yes, I thought they moved to MSN search @ Q2/06 .

Smart people at Amazon. Their search engine is very good, almost as good as Google.

Well, the first results are OK, but they have problems with duplicates.