Updates to be found in Microchip Forum:
1. PUF using sdcc : http://vasco.gforge.enseeiht.fr/
"PUF is a USB application framework dedicated to Linux (on the host side) and to the PIC 18F4550 family of microcontrollers (on the device side). PUF includes: a bootloader that can flash the PIC application through the USB, a PIC demo and its corresponding application on the host side, and docker, the host-side flash programming utility.
2. PIC18F4550 USB/MSD sourceforge, Apache License V2.0
3) GPLed USB stack with SDCC
4) PICDEM FS USB bootloader for Linux/Windows using libusb
5) PICDEM FS USB demo appliation under Linux/Windows using libusb
6) PIC18F2455/4455 Assembler USB Firmware (MPASM) From Bminch
7) MPLAB C18 based PIC USB firmware from Bminch (simpler than Microchip's)
8) Microchip USB C18 Firmware framework ported to SDCC
9) USB Midi (using gpasm and sdcc). It is said that the firmware is also based on the Microchip Firmware Framework.