Google Sitelinks Patent
On some top search results, Google adds additional links below the extract that point to subsections of the top ranked Web site. The official name from Google for these subsite links is Sitelinks. See below for a screen shot of Sitelinks for ALA.
Google has submitted a patent application, #20060287985, "Systems and Methods for Providing Search Results" for the algorithm that selects those Sitelinks. Bill Slawski dissects the patent application to find more details on how it works. This is, of course, assuming that Google is still using the methodology described in the patent application, but it does seem to connect with the examples from Google that I've seen. As Bill writes, "The patent does note that it is only the top result they are showing these additional links for." He also discusses how pages are chosen to have Sitelinks and what determines the ordering of the links.