Google is about to make a new revolution. After seeing that Google has word categories, a new feature shows that Google works on a new approach to search: a semantic web without a special markup language. The new feature allows users to refine the search by choosing directions to follow. If you search for [tired], you may want to find medical treatments, or alternative medicine suggestions, research papers or news.
If you click on [Treatment], Google will follow this link: http://www.google.com/search?hl=en &q=tired+more:condition_treatment &cx=disease_for_patients &sa=N&oi=cooptsr&ct=col1&cd=1
Google uses a new operator, more, that has special arguments (in this case, condition_treatment). cx is a new parameter that provides the context.
You can view the live Google search here and more comments at Nico's Blog, that thinks this is search results clustering. I think it's not just clustering, it's understanding user's intentions and providing a better experience.