Many different types exist and they perform different I guess..The type I use I from a car parts shop
and is often used to reduce engine noise and other unwanted sounds in car hifi systems.
Inside the bottom of the case is mounted some, to avoid sound refelctions, and also increase mass.
On the back side of the motherboard metal plate, is also mounted some,
it is about 12mm thick and will just touch the cabinet plate,
now the sound is better if you knock a bit on that plate you hear the difference..
Inside the case lid just over the CPU FAN is also mounted some..
The total sound level is a bit lover, but it is really not that big a hit for the money, sorry.
I have also measured a temperature rise on the CPU at about 2C due to the lower air movement on the rear plate I guess.