I was out searching some of the Google Trends terms and noticed that one of the search results pages provided me a nice little search box that would allow me to directly search that site without having to visit it first.
By searching a specific term in the new site specific search box, you are taken to a different set of results that shows only pages from that specific site. In addition, it also puts the advances search query in the search box allowing you to make easy adjustments and continuing searching that specific site.
I was a little shocked at first, what with the whole direction Google took to get your search results out of their search results, but the overall experience is positive and makes this an interesting, yet nice, addition to the standard search results pages.
Edit: Google Blog has an explanation on their site here.
[O]ver the past few days we have been testing, and today we have fully rolled out, a search box that appears within some of the search results themselves. This feature will now occur when we detect a high probability that a user wants more refined search results within a specific site. Like the rest of our snippets, the sites that display the site search box are chosen algorithmically based on metrics that measure how useful the search box is to users.