Seadragon enables smooth browsing of images as well as Deep Zoom experiences.  It’s the power behind the PhotoSynth technology I mentioned earlier. To operate a Deep Zoom experience like this Michael Jordan one I like, you click where you want to zoom in and shift-click to zoom out (you can also use the mouse wheel).  There is a ton of great stuff in the Seadragon showcase (of course I’m partial to the shipping containers).  Enjoy!