AVR-USB stack from Objective Development

Objective Development Software GmbH develops USB stack for AVRs. It is an Open-Source project, with free avr-gcc.

AVR-USB is a firmware-only implementation of a USB 1.1 compliant low-speed device for Atmel's AVR microcontrollers. It runs on any AVR microcontroller which has at least 2 kB of Flash memory, 128 bytes RAM and can be clocked at 12 MHz. No UART, timer, input capture unit or other special hardware is required (except one edge triggered interrupt).
AVR-USB can be licensed freely under an Open Source compliant license or alternatively under a pay-per-unit commercial license.

They also give project-examples. I think this is cool: Wireless thermo- and hygrometer with up to 16 sensors and a central logging facility accessible via WWW.