USB Sniffers

USB Snoopy, a tool to watch device interactions on windows.
SnoopyPro, a spin off from USB Snoopy.
usb-robot, a user-mode driver for devices without one.
Windows program to send simple commands to USB serial devices.
usbfs_snoop, kernel 2.6 based snooping.

Tools for Running USB

usbutils (Greg Kroah-Hartman)
USBView (Greg Kroah-Hartman)
usbd (user-space daemon) (Thomas Sailer)
usb-printer-id to read back status information (Adam J. Richter)
Linux Hotplugging, including basic scripts to set up and run USB.
USBMon A prototype Architecture for USB monitoring under Linux (Dave Harding)
usbtree A utility to show attached devices

User Mode USB APIs

libusb (Johannes Erdfelt)
Java USB for Linux (David Brownell)
libhid: USB/HID support library, based on libusb (Martin Krafft, Arnaud Quette and Charles Lepple)

Video camera tools

camstream application software
Creatix webcam info/HOWTO
vgrabber sample V4L program
Video4Linux project
Video for Linux resources
VLSI Vision CPiA for Linux (parport & USB)

Other Device Related Tools

Casio KP-C10/50 label Printer application.

Hardware tools

The EZ Protoboard for prototying Full Speed devices.
Downloader for ezusb2131 boards.