Google may be the best search tool on the planet, but it's still missing a very useful feature--the ability to preview a site before you visit it. A simple text listing and site description usually isn't enough.
That's where Firefox extension Google Preview comes in. It displays a thumbnail of each site to the left of the description in the Google search results. That way, you can see if it's a site you want to go to. The add-on also offers site thumbnails when you search using Yahoo.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development. This link takes you to the vendor's site, where you can automatically install the file into your Firefox browser.