Ajax @ Amazon?
Given that I was one of the lone attendees without a demo to show at the Ajax Summit earlier this week, I wish I had known about Amazon's new Diamond Search, which uses a combo of DHTML and Ajax-style backend communication to allow users to narrow search results in a purely visual way. Move the sliders back and forth and watch as some mysterious process fetches the number of search results in real time.
Full disclosure: I didn't build this and have no idea how it works under the hood. A press release went out last Wednesday and the app has been publically available for at least a week. News to me.