It's known that hardware sometime is expensive and most of the time we buy things that we already own. That's why i'm writing this: i had 2 PC broken by a lightning and one that stopped working by itself for unknown reasons, but this was very very old, so yesterday i have dismantled these old PCs. Most of the pieces were old and not compatible with newer desktops except some hardware like heat sinkers ,fans and some cables. Most of the fans on desktops works from 0.5A to 0.1A at 12V so by knowing your hardware you can add them to your desktop.
While i was disassembling the last pc, i found 3 really useful fans and a great heat sinker for my Raspberry Pi:
This is the heat sinker
and the 3 fans.
Now the heat sinker can be added without needing any skills; you just need it and the thermal paste.
Fans: first you need to check how much current it will draw with 5V (it MUST draw less then 300mA on Rev. B and less then 500mA on Rev. A according with the Wiki page http://elinux.org/Rpi_Low-level_peripherals#Power_pins).
second thing to do is checking the GPIO pins that you need:
Be very careful with the 5 V pins P1-02 and P1-04, because if you short 5 V to any other P1 pin you may permanently damage your Raspberry Pi.
then you can simply use some cables taken from the old pc connectors (like jtag cables) to connect your fan to your Rasp Pi.
As you can see it works perfectly on my Raspberry Pi :)
according to the "vcgencmd measure_temp" command, the temperature of my raspberry after 3 hours of works (i use it as a torrent client) is temp=27.7'C, pretty low :)
For those asking what fan i have used, mine is this one:
DC 12V, 0.45 A.
as always, sorry for my bad english, i'm Italian :D