As John says, this certainly opens up entirely new classes of search engines and other applications that can be built by leveraging this data.
Alexa is charging for its data, but it isn’t much.
The first 10,000 requests per month are free. Thereafter, requests are charged at a rate of $.00015 each (just 15 cents per thousand requests.). For example, if you make 100,000 requests to the Alexa Web Information Service during a given month, you will be charged $13.50. Your first 10,000 requests are free, while your remaining 90,000 requests are charged at a rate of $.00015 each: .00015 * 90000 = $13.50.
This is a developing story and some of the links are not live yet. More on this as things progress.