I have been working on a Reprap firmware that will be used on Arduino Due, long story short, I ported/created a few low level driver in Chibios/RT.
This is the forked OS, and is based on 2.5.2.
This is the firmware code and demo how it is used.
LLD implemented: ADC, DMAC, GPT, Peripheral DMA (PDC), USB, PWM, Serial, UART, SPI.
Bonus: I hacked a USB mass storage for MMC/SD card (SPI) together and fixed a few bugs there too.
In the firmware I hacked part of the LUFA and created a USB composite device that gives 2 COM + 1 Mass Storage.
The code is absolutely alpha quality but should give you a starting point if you are playing with it.
Absolutely feel free to report bug/fork, or help me to port to 2.6 =)
Moderator edit: Moved to a separate thread.