My computer was very loud. It was beginning to annoy me. I have seen this discussed on slashdot, as well as many other places. Most suggestions seem to boil down to: buy xxx expensive and rare part from yyy and things will be better. Their suggestions may be true, but I am a cheap bastard. I did come upon one page that recommended making a box out of plywood and lining the inside with carpet padding and sticking your case in there. That would probably work better than what I am about to describe, but in addition to me being cheap, I am also a lazy bastard.
I bought a two square yards of carpet padding ($1 at HomeDepot) and a bunch of hot glue. I carefully lined the inside of all of the panels of my case, an InWin full tower, with carpet padding. Then, I started applying it everywhere else like a lunatic until I ran out of hot glue. If you weren't concerned about your case being butt ugly, I think that applying it to the exterior would yeild a further reduction in noise. I didn't do that.
You may think this is a stupid idea. You may be right. If my computer blows up, I'll let you know so that you can stop before it's too late.