Dilshan Jayakody has written an article detailing his open hardware project the USB random number generator:
This project is about USB port base true hardware random number generator and it is design around the avalanche noise which occurred in reversed biased P-N junction.
This random number generator is design using commonly available electronic components. The core component of this system is Microchip’s PIC18F2550 microcontroller. The noise source is 2N3904 (Q1) transistor and LM386 low voltage power amplifier is used to amplify the incoming noise signals. This peripheral is design to work as a self-powered USB HID device.