I downloaded 2 major add-ons for Firefox today:
Cooliris, which is a full-screen search that shows your results as a scrollabe 3D wall. Effective with Google Images, Yahoo Images, deviantART, Photobucket, YouTube, and Flickr.
And Nioa 2.0 eXtreme theme. It's a really neat interface, but it looks better with Vista than it does XP. It gives options of background themes, rounded tabs, and other stuff.