This project uses an ATTiny461, but should work with other AVR:s as the Tiny45 or 25. One pin on the CPU is used as a A/D, and to this pin a potentiometer is connected. The PC software uses libusb and amixer. It reads the potentiometer value from the AVR continuously and adjust the systems volume according to it's position.
This device is powered from USB, no external powersource is used.