Sniffing

• lower-level drivers
  • linux kernel module usbmon (writeup
  • libusb
  • usbsnoop.sourceforge.net (based on Benoit Papillault's USB sniffer) (Windows)
• apps
  • Wireshark can analyze USB too!
• more
• hardware
  • expensive shit

route USB packets over network

• usbip.sourceforge.net (pre-alpha)
• USB Redirector
• USB Over Network

Websites with the largest amount of useful technical info

• BeyondLogic.org (see "USB" drop-down menu)
• USB Central from Lakeview Research — TONS of useful links
• USBman.com
• EverythingUSB.com

Writeups

• USB protocol: one, two
• practical USB terminology (oh so very helpful)
• USB in Linux: [1] [2]