After testing a persistent header, Google continues to experiment with infinite scrolling for Google search results. A Webmaster World user spotted a new box that replaces the standard pagination links: "show more results". When you click the message, Google loads the second page of results below the top results.
Barry Schwartz says that Google tested a similar interface back in June. I remember that SearchMash, Google's old playground for search experiments, used infinite scrolling in the first iterations. Last year, Google Image Search added infinite scrolling,
There are many extensions that add infinite scrolling to Google search results pages. One of the best is AutoPager, which is available for Firefox and Chrome.