Host Side USB Drivers
www.libusb.org - An abstraction library for controlling USB functionality on Unix (Linux, BSD, Mac OS X, etc).
sourceforge.net/apps/trac/libusb-win32/wikis - A Windows version of libusb with the same API. It requires the installation of a kernel DLL which is part of libusb-win32.
www.icsharpcode.net/OpenSource/SharpUSBLib - A C# wrapper around libusb-win32.
Other Firmware-Only USB Drivers
mecrisp.sourceforge.net/mecrimus-b.htm - Mecrimus-B is an assembler-only USB implementation similar to v-usb, but for the MSP 430.
code.google.com/p/16fusb - 16FUSB is an assembler-only implementation of low-speed USB on PIC microcontrollers.
ziblog.ru/2014/02/22/programmnyiy-usb-na-st... - This is a firmware-only low-speed USB device implementation for the STM8 microcontroller. The link points to a blog in Russian containing the source code download. Use Google Translate to translate into your preferred language.
scanlime.org/2010/04/its-alive-bit-banging-... - This is a full speed USB host controller for the Parallax Propeller microcontroller.
www.cesko.host.sk - This is Igor Češko’s original work. Although it has given the inspiration for V-USB, the two projects share no common code.
www.xs4all.nl/~dicks/avr/usbtiny - This is Dick Streefland’s driver which is derived from an earlier version of V-USB. Dick’s driver has less options and the code is therefore easier to read.
Driver Firmware for AVRs with USB Hardware
www.fourwalledcubicle.com/LUFA.php - This project is a driver for Atmel’s AVR microcontroller series with hardware USB support and some of the AVR32 chips, released under the MIT license.
savannah.nongnu.org/projects/avrusb - A freeware USB driver for the AT43USB3XX series of chips. The project seems to be dead, though.
Embedded USB Hosts
instruct1.cit.cornell.edu/courses/ee476/Fin... - This is a firmware-only low speed USB host for AVRs.
www.vinculum.com - Their USB host chip is an easy method of adding USB host functionality with little effort.
www.embedded-projects.net/index.php?page_id... - This USB host driver supports Atmel AVRs with built-in USB hardware and the SL811HS chip. It includes class drivers for various USB classes.
www.sxlist.com/techref/io/serial/usb/atapch... - A firmware-only low speed USB hardware analyzer.
sourceforge.net/projects/usbsnoop - This is a USB sniffer for Windows. It is of great value when debugging firmware problems in a USB device.
codeandlife.com/2012/01/22/avr-attiny-usb-t... - A step by step tutorial introducing V-USB
www.usb.org/developers/docs - The USB specification. Not easy to read, but the definitive reference.
www.beyondlogic.org/usbnutshell/usb1.htm - "USB in a Nutshell" is a good overview of the relevant parts of the USB specification. Read this instead of the real spec!
www.lvr.com/usb.htm - Many good links related to USB.