Here is the fan modification that I did to one of the Raspberry Pi's.
60mm 5 volt fan (EverCool EC6015M05CA from FrozenCPU)
60mm rubber fan silencer (from FrozenCPU)
USB to 3 pin molex fan header cable (off eBay)
Here's the fan and silencer. It comes with 4 self-tapping screws
Tools. Yes you'll need something to put a hole through the top of the case. I bought a 60mm drill bit normally used for locksmiths to drill into doors. As you can see it works on plastic Raspberry Pi cases too.
The center drill bit is designed to pull the blue part of the bit into timber, however it got stuck before the blue part had even reached the plastic. I had to drill out the center hole with a normal drill bit so that the blue part can touch the top of the case.
Here is the fan mounted on the top of the case. That is a resistor cable with the blue plug. It came with some 12 volt fans. I was thinking of using them to reduce the fan speed (the fan is rated at 4000 RPM) however they don't work at 5 volts.
Another shot side-on so you can see the fan mounted on the top of the case
A shot from inside the top of the case.
These are the USB to molex fan header cables that I got off eBay.
And here we are fully assembled
Another shot at a different angle of the completed modification
The fan is blowing air into the case. This was because if the fan were turned upside down it doesn't mount properly. The holes on this side of the fan are countersunk. I think it provides more airflow over the 3 main heat producing components on the Pi this way, rather than trying to suck the air out of the case.
The rubber fan silencer is useful. The fan itself isn't exactly 60mm so there is a gap between the hole in the top of the case and the fan if mounted directly. It also cuts out the vibration which was the reason why I got it.
I did buy a couple of chrome fan grills but the screws aren't long enough to hold the fan on as well as the grill. If I can find some longer self-tapping screws I will try and mount the grill later.
Running the fan off the Pi's USB port means the fan doesn't run at full speed which makes it quieter and should help it last a bit longer. Just make sure your power supply can provide extra power to the Pi.