Imagine searching inside Wikipedia or Amazon.com, but on the Google search site. Google's got a test going of a way to search inside different sites from within a Google Web search results page. If you type in "amazon," "wikipedia" or "new york times" on Google, it brings up the relevant result at the top and immediately underneath is a second search box. If you type a keyword in that secondary search box it will return results from within that site.
(Via Search Engine Land)