Vendor dependent firmware
Basically you can get firmware source code from your chip suppliers, such as NXP, Atmel, ST, Freescale, Microchip. The code bases are very informative. Sometimes the source code is not public available. Then you have to look for general purposes resources for firmware development.

usbn2mc is an universal USB Interface for a Microcontroller which has 12 I/O Pins, a SPI Interface or a DMA Controller. The project is based on the USB Bridge from National Semiconductor USBN9602/ USBN9603 / USBN9604. The Advantages of this USB-Controller are the price, the configuration and the availability in many electronic shops. The main part of the project, is the common usb layer of the controller, which manages the communication with usbn960x. It is written in ANSI C, this means it compiles with every ANSI C Compiler.

Intra2net has a software collection for FTDI's FT232BM/245BM in bit bang mode. libftdi - A library (using libusb) to talk to FTDI's FT232BM/245BM, FT2232C/D and FT232/245R type chips including the popular bitbang mode.

A USB example for PIC18F4550. Connect your PIC18F4550 into a PC using a USB cable. A virtual RS232 COM port is created, allowing your application to print RS232 messages into HyperTerminal. Allows easy porting of legacy code that used RS232 comms.
This project is a modified form of the CDC (Communications Class Driver) available from www.microchip.com. The www.htsoft.com forum has some discussion on this code.

The PIC 18FUSB online website shows how to use Microchip 18FXXX microcontroller USB Bootloader and Learn PIC USB with a minimal USB application.

The eegeek has a good article written by Evan on Understanding the USB firmware for the PICDEM full-speed USB demo board.

This is a wiki page of development USB device based upon Microchip PIC18F microcontrollers.