Ask has been doing some good stuff recently, particularly with their Binoculars option. Previously you could just run your mouse over them and you'd get a thumbshot of the page. Now, as a result of a tie up with Compete we get more statistics, such as the number of visitors to a site. This can be really useful when it comes to deciding which sites to visit, or how authoritative they are (with the obvious point that lots of visitors does not a good site make) very quickly. I've got a screenshot below which is two images into one, so that you can see both options available.
There are other search engines that offer thumbnails of course, such as Exalead and sites that offer statistical information, such as Alexa but this is a really neat and effective way of bringing all that information together.