Google has added a new feature in which a secondary search box that appears within some of the search results themselves. See, for example, the search results for a “NASA” search.
Note the “Search nasa.gov” search box below the link. According to Google,
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.
I’ve not seen this yet for any other search I’ve done – have you?
Source: Search Engine Watch
Update: 3/12/08 Looks like Google within Google also appears for some university names, including the University of Wisconsin. I also saw it for the IMDB (Internet Movie Database).