This is a simple and really cheap way to control pwm computer fans. (4-pin connector)
It also allows to read the tacho value.


Finished:
-Hardware
-Python script for adjusting and reading the speed

TODO:
-Get sensor values from mainboard and disks, then adjust fan speed accordingly.


Hardware:

There are some schematics in the hardware folder. If you have any questions don't hesitate to contact me.

The hardware is based on the V-USB project (http://www.obdev.at/products/vusb) and uses their design with zener diodes (with-zener.png). No additional components (except for pin headers) are needed. 
I used an Attiny2313 because it has 4 PWM channels and you can configure interrupts on PORTB, which are used for tacho detection.
The design should work on bigger ATmegas.