Official Google Blog: Beyond Instant results: Instant Previews

With an overview of your search result....that highlights the most relevant sections, you will find... that just right page as quickly and easily as flipping through the pages of a magazine. Just click once on the magnifying glass next to the title of any search result....
and a visual overview of the page will appear on the right.
Then just, hover your cursor over any other result to see....
your preview.