It's Google's "I'm Feeling Lucky" for the year 2005. You type in a search phrase into http://instant.search.yahoo.com/ and watch this contextual bubble overlay the bottom portion of the page, within that contextual bubble you find the number one result for your query (sometimes). Here is an example of the number one result for the search phrase, "search engine", at Yahoo!.
Danny has a large write up at the Search Engine Watch Blog and I started a forum at the Search Engine Watch Forums.
