Tutorials
Here are some tutorials. Some of them are prepared by myself (projects prepared by me may require AVR Project IDE).
How to use USnooBie with Arduino
A step by step tutorial on how to build a RFID tag reading USB keyboard using the USnooBie
USnooBie's USB HID Report Descriptor Tutorial 1
USB Wii Classic Controller - Instructable
My "Getting Started with LibUsbDotNet" page (video and example project). I also have an example project "USnooB + Optical Mouse Sensor" here.
SparkFun's USB Touchscreen Mouse
Connecting Nokia 3310 LCD to USB using AVR
AVR General Tutorials
AVR Programming Introduction - Hack a Day, there are several parts.
AVR Tutorial, Step-by-step how to start AVRs - Ladyada
A Quickstart Tutorial for ATMEL AVR Microcontrollers - I Make Projects
AVR-GCC Tutorial - Scienceprog.com, a collection of tutorials about using AVRs
AVR tutorials - AVR Freaks, a collection of tutorials about using AVRs on the AVR Freaks forums
Tutorials on SparkFun, the "Beginning Embedded Electronics" section focuses on AVR microcontrollers.
Projects
Turn USnooBie into an USBasp, make your own USB in-circuit programmer for Atmel AVR controllers with an USnooBie
See a massive list of V-USB projects, check this out for examples and ideas.
Stealth USB CapLocker, neat little prank using a tiny PCB that inserts right into the USB port.
Haunted USB Cable, same as the Stealth USB CapLocker, but hidden inside a USB cable.