V-USB is a software-only implementation of a low-speed USB device for Atmel's AVR® microcontrollers. A tutorial can be found on my site: http://www.workinprogress.ca/v-usb-tutorial-software-only-usb-for-mega-tiny/
This zip contains a firmware that receive and send to pd. There's also a external that use pthread for receiving from the firmware and send using messages and lists.
A good starting point for making custom electronic instrument at a fraction of the price. If you're looking for something like the arduino, I have a custom board based on Atmega640 called HiCu (62 i/o, 16 analog). You can find the source (schematic, PCB, bom) here: http://www.workinprogress.ca/projects/hicu