USB Implementers Forum (USB-IF)
This is the official site of the creators of USB technology. All the USB-related specifications can be found on this site.
USB-IF Technical White Papers
Technical "White Papers" on various topics written by the specifications authors and other industry experts.
The official USB specification provides technical details to understand USB requirements and design USB compatible products.
Microsoft Windows USB Architecture
Good starting point for everyone who are developing USB devices and drivers that work with Microsoft Windows operating systems.
Windows Driver Kit USB Reference
Describes the specifications of I/O requests, support routines, structures, and interfaces used by Microsoft Windows USB client drivers.
USB Central by Jan Axelson
Information, tools, and links to material about the USB.