Last month, Google announced that it is now showing a new and improved sitelinks search box within search results for some sites. Its aim is to help users find specific content on third-party sites more easily.
The box is now more prominent, and supports autocomplete.
Webmasters can markup their sites so Google has the ability to display this in search results for their brands. There's also a way you can specifically tell Google you don't want this to appear for your site.
<meta name="google" content="nositelinksearchbox">
I guess this way, you can encourage users to actually use your site to do the searching. Either that, or it will encourage them to get more specific with their Google search. I'm not sure how often these boxes are being used anyway, but it must be a substantial enough amount of people for Google to even offer the feature.
Image via Google