Search Engine Land spotted a new Google search feature. If you click on a search result from a forum and then go back to the search results page, Google adds a link that restricts the results to forum pages: "get more discussion results". The same feature is also available if you click on "show options" and select "discussions", but it's more difficult to find.
It's the first time when Google changes the search results page after clicking on a result. Google could add other related features: for example, it could show a list of similar pages.
When you click on a search result, Google tracks the click and uses it to adjust the order of search results. An interesting approach to personalize search results is to use the clicks to disambiguate the query and dynamically adjust the results. Surf Canyon is an extension that customizes your search results based on the results you've selected and shows recommendations when you click on a search result.