LUFA (formerly MyUSB), the Lightweight USB Framework for AVRs. This project is an open source, MIT Licensed USB stack for the USB enabled 8-bit AVR microcontrollers. For more information, visit the project home page from the links bar to the right.

Official stable releases are available from http://www.fourwalledcubicle.com/LUFA.php - the SVN repository here is a synced mirror of the LUFA development repository used by the author and is thus highly unstable and should be used for preview purposes only.