I managed to get hold of a load of that corrogated foam that they use in music studios. I cut it all to size and fitted it on more or less every bare panel inside my server case (I have this on in my bedroom while trying to sleep!) It does make quite a difference but it only seems to get rid of the higher frequencies. I've also placed the case on top of some of this foam and put a slab of it around the back of the case on the adjacent wall to deaden the rear exhaust fans. Damn I'm a geek!

The best idea i have seen it to: First remove all the fans, secondly, completely seal your case with silicon. Now all you have to do is fill it with cooking oil and your set. 100% silent and cool. And apparently it works really well!

foam is also an excellent thermal insulator. Be careful as you will trap heat normally escaped through the case itself.