In addition to the products linked to in ces' post, there is also the Nexus Drive-A-Way. I used this to silence a Samsung Spinpoint F1 750GB with excellent results. It virtually eliminated airborne noise, although it did nothing to reduce vibration. I had it resting on foam to fix that problem.
A minor issue: I needed to fiddle around and fit the cover on loosely. If it was on too tight then the airborne noise would become audible. Once it was adjusted right, it worked beautifully. http://www.youtube.com/watch?v=DkAAgquVmbk