I haven't tried it myself yet but it might well usefully attenuate the last vestige of high-frequency noise coming from the PC.
 
The foam matting at that website doesn't look the same as that now sold by QuietPC.com. Scientifically-speaking, it's no good putting just a single layer of a foam on the case's interior. For effect, the material needs to be composite, ie layers of different density. Possibly, QuietPC.com's product is better in that respect.
 
As it happens, I'll be personally visiting QuietPC (UK) next week and I'll be taking a look at their sound-deadening composite foam. It's more expensive, I think, than the material you've already seen advertised.

I use automotive car stereo stuff called dynamat works well and cuts Vibration also and sound.
 
there are similar kinds these are asphalt based.

theelviscerator,
 
You may well be kidding yourself about the benefit of using automotive sound-deadening pads. Okay, if you've absolutely no other sound-quietening measures employed inside the PC case, then such pads will attenuate some of the low frequency stuff, mainly the LF boom from the sidepanels, but acoustics theory shows quite clearly that these are no good for middle and HF frequencies and for much of the air-borne noise you get from a PC. I speak as a scientist/engineer and I've also discussed this with a friend of mine who's a professional air-conditioning engineer who's used proprietary padding on all manner of systems over the years.
 
I did, in fact, try those bituminous automotive pads myself about 18 months ago and they made no difference whatsoever. They simply don't attenuate those irritating mid-frequencies. For that, you need a composite pad, of different densities.

Another thing you should carefully consider before adding any sound-deadening foam to the inside of the casing is that, by doing so, you will considerably decrease the case's ability to dispense with heat by radiation. In other words, you'll thermally insulate the inside surfaces of the case and prevent them from absorbing the excess heat generated by the CPU, memory and other heat-generating components. This is usually more of a problem with Pentium 4 and Athlon systems. Normally, the casing metalwork, especially near the top, acts as a heat dissipator. This happens by default, not especially by design, although some case manufacturers are now cottoning on to the use of lightweight aluminium cases, as they can dissipate (radiate) heat at a greater density than steel ones. I'd say: Add acoustic foam (the right kind) to the bottom and sides of the case but leave front, back and top untouched.

Have a look at this comparisson of a couple of different types: http://www.bit-tech.net/review/136/
 
The general consensus was that it did help a bit, and that temps went up a bit.
 
At the end of the day it's a compromise between noise and temps (as always).
 
You may be better off getting a baybus or something similar. You can then leave the fans on high power when you're benchmarking or playing games, and turn them down to low power when playing music/films.

I use Dynamat, and it helped a ton with fan noise since the I have several fans and rather large panels in this case. And no, I can most certainly say that I am not kidding myself as to its performance (I have done some enclosure design myself, and controlling the environment was something I had to learn). As my case is so large, the panels had a tendency to resonate with the fans and the pads make the panels more "viscous" and quiets it down (especially when either my CD-RW or DVD-ROM spool up, and I don't hear my HDDs at all anymore). When I had my P3 with a Delta fan on it, the material made the use of the fan bearable as the whine from it is incredible. As for acoustic foams, most of the time you will see this used to vary the "q" (trying to remember here) of an enclosure and synthetically increase the bass response of a driver/enclosure combination without having to make a larger enclosure.
 
After looking at that review Bursar posted, I would definately go with the lower profile material since it will take up less interior space and not interfere as much with airflow (although the temp differences didn't seem to be a problem).

Clutch et al, I think you've missed the important points I was making.
 
Basically, what I was saying was that that dense bituminous automotive material will reduce only low-frequency noise (and frankly there isn't a lot of that in a PC, it's mostly mid-frequency stuff). This is why those pads were originally designed for car panels - they stop panel-boom.
 
Foam materials, ie fairly low-density stuff, will certainly help with attenuating the mid and high-frequency noise. If you've ever seen the inside of a professional acoustics testing chamber, you'll realise that this is so.
 
Sure, it'll help a little to add those bituminous pads, especially if you'd previously not already significantly reduced the noise by employing a quiet PSU and quiet fans but, if you've already done that (as in MY case), adding the bituminous pads won't make a jot of difference.
 
I agree, it's all a bit of a compromise between cutting down the noise to an acceptable level and allowing the cooling to do its job. Do bear in mind that people's perception of noise varies. What's acceptable to some isn't to others, especially at low levels. For my own sins, I've got very sensitive hearing.
 
The really neglected part of the 'quiet engineering' of PC cases is the physical mounting of all the rotary parts into the casing. It's the sound conduction of their fixings into the metalwork of the case that's one of the biggest problems and, until that's seriously addressed, there'll always be a vestige of irritating sound left.

The easiest way of fixing that is surely to get some rubber gromits to lessen the vibration.

The material is actually designed to reduced panel rattle, and to lower the overall assembly's resonant frequency (assembly meaning panel and material). Since the vibration of fans and drives and can make surrounding materials vibrate, lowering the resonante frequency of such materials will keep the vibration from getting amplified through said material. GM did such research on the topic when building the Aurora and found that many people loved the German sedans (Mercedes, BMW) because they had a deep "solid" sound to them. In their research, they found that the collective resonant frequency was lower on these sedans than on a typical GM vehicle. So, they concentrated heavily on chassis design and supplemented extra areas with materials like this to reduce audible rattling (mid-frequency noise, normally tuned-out by most people) and vibration. So, scientifically speaking, there is good reason to use material like this to reduce noise induced by fans and drives (if the need is that great).

To Bursar - wrong!
 
Leaving ANY metallic connection whatsoever between the fan and the casing will cause the rotary noise to be conducted into the casing metalwork, and normally at least one screw of some sort is used for that. Grommets will make virtually no difference; they're too dense a material.
 
I've played around with all kinds of fittings and haven't been satisfied with a single one. In the end, I've stopped using the fan (a chassis fan, not a CPU fan!). I'm currently working on an idea for a fan 'cushion' that wouldn't involve any sort of metallic or high-density connection to the PC casing.

That's me corrected then

Bursar - Yup!
 
Actually, although I've been banging on a lot about acoustics principles, it's amazing the effects that quite minute changes can make, inside a PC case. You really only find this out by experimenting on the job. Sometimes, for instance, the minor displacement of a rubber grommet can change the noise level quite significantly, not necessarily improving it and, quite often, worsening it!
 
I think the answer in the end, when it comes to chassis-mounted fans, will be to completely mechanically decouple the fan unit from the PC casing.

That could be done by using a form of "motor mount", which is commonly used for engines and transmissions in automobiles. Essentially, you would be using a grommet of sorts (as Bursar recommended), but with no metal hardware linking the motor to the frame (or in our case the fan to the case). One bolt fastens the rubber mount to the case, while another fastens the fan to the mount. With a soft enough compound this would work for some chassis fans, but not the PSU, CPU, or GPU fans most of the time (there is one massive flower-type heatsink that doesn't have the fan mounted to it and could benefit from this, but the fan is large and quiet already so it probably wouldn't be needed).

No, the materials involved would still be too dense.

How do you mean? CD Changers have used various mechanisms to decouple the playback unit from the chassis so the outside world doesn't affect it, so I don't see why this couldn't be done in the same way.