I might be a bit behind the times with this one, not sure.
Ive just found a neat little add on that speeds up web browsing.
Its called Cool Previews and can be found here
Cooliris, Inc. | Beyond the Browser
This add on makes an icon appear when you hover over a link or image link, click the little icon and you get a preview of the page for the link. So now if I see a thread here I want to peek at I can get a preview without having to go into the thread or open another tab etc.
Makes Google searches much quicker, you dont have to go to the page to see if its what you want, you can preview it from the results page.
Its cross platform so it works on Mac,Win and Linux.
Any other useful add ons you come across post them here!