Google has implemented a while back, something we named Google Search Web Categories, basically, those little sub links you get under the URL to take you to other popular sections of a site. You can see an example if you search on search engine watch. So how does one earn those links? That is the topic of discussion at a thread in WebmasterWorld.
The answer? Well, no one really knows exactly. I would assume you would require lots of repetitive linkage data to the core sections of your Web site. For example, check out apple and you will notice the web categories for apple are "iTunes - Download iTunes - iPod+iTunes - QuickTime - online". Those are probably the four most popular sections on the site. Was it done manually? I doubt it, why would they use "online" to symbolize the Apple Store? So I believe it is based on linkage data.