Google has recently launched a new "onebox" feature, where a site gets not only their sitelinks and a stock quote if they have one, but also a search box that searches the site.

Here's an example:

Whilst the technology to do this search isn't ground breaking (it does a site: search with your search term appended), it is interesting that they have chosen to do it.

According to Google, it is an extension of the Sitelinks and deals with secondary searching of a site. So those sites that people search for and then search again using that term. Apparently, it came out of their study of "teleporting" - a name they use for the user behaviour of searching for a company name. 

Again, like Sitelinks, the need for them is generated algorithmically, based on the liklihood of a user doing a secondary search.

Interestingly though, it's adding more "clutter" to the Google interface, and pushing the second and subsequent results further down the page, which is an interesting trend that has been occuring more and more of late.