Firefox with Greasemonkey: User script Google Fx adds a ton of features to the Google search interface—from related searches and Wikipedia results to thumbnail previews and search suggestions.
Using the script is fairly obvious, just install it and search for any term in Google—you'll see all of the added functionality immediately. The added features include related searches, Wikipedia results, thumbnail previews, Google suggestions, image lightbox, auto-paging, additional search engines, and more, but it's the integrated settings dialog that really makes this extension interesting—you can use the user script commands menu under the Greasemonkey icon to pull up the Google Fx settings, and turn features on or off to suit your own preferences.
Google Fx is a free download, requires Firefox with the Greasemonkey extension. For more Google customisations, see how to add the Google Reader unread count to the FavIcon, or Internet Explorer users can check out Customize Google for IE.
Google Fx [userscripts.org]