I have a working prototype of the above circuit that controls the speed of my Laing DDC water pump. Using PWM from the motherboard, output voltage can be controlled from 7.5 to 12 volts (Speedfan or your BIOS controls the signal). This means your water pump (or non-PWM fan) runs quieter when your computer isn't doing anything.
You can hear the noise benefits in my system here:
mp3 audio
The kit (assembly required) would be around $15, perhaps less depending on the interest. Any takers?