A month ago Google began testing page previews in the web search results. Yesterday, Google officially added them for all users. If you do not see them yet, then use this link to try them out yourself.
Here is a picture of it in action:
Here is a video demo by Google:
Some history, in June 2004, Ask Jeeves went with binoculars on the SERPs, which is pretty much a similar concept. Google reportedly tested this concept in August 2004 (or it may have been an extension). But Google did give searchers an option for page previews in November. Plus also check out the post at ResourceShelf for more history.