This is my Personal Favorite. This 'Time Savior' Add-on lets you to preview your desired links without leaving the current page. Just put your mouse over any link then on the blue Magnification Glass icon then the preview of the link automatically shows up without you leaving the page.
Try it on facebook; I actually loved it on Facebook. Searching for friends is easy with this.Here are its features. It works with every website(google, facebook, forums, flicker,blogger,...). Do write if it doesn't.