At first, I bought passive cooler for CPU. It was very cheap (2.24$), from DealExtreme.
|Passive cooler on CPU|
That cheap coller lower the temperature about 8°C. It was pretty good, but not enough for me. I've found an old ArcticCooling fan. At first, I've soldered it with 12V DC power adapter. It was very loud. Everyone want's silent HTPC, not loud like blender. After that bad idea, I've grabed old USB cable and cut the mini USB end. USB port has 5V voltage, so it is really great for powering fan at low speeds. After you cut the cable, you need only red and black wire. Red is +5V and black is 0V. Solder red wire from fan with red wire from USB cable and black with black. Now you have fully working USB fan.
After all that work, I've had only one problem. How to mount it to the Pi. I've grabbed 4 screws (M4) and attached them to the fan. They are high enough for fan to not to touch Pi. I used a duct tape on them for sure, someone can accidently put them on GPIO or something on Pi and you know what can happen.
Now my Pi is running at 23°C and is absolutely quiet. Next I am going to make my own aluminium case from old FDD holder. Stay tuned.