Make:

This is exciting, well for some, GPL'ed AVR USB! - "USBtiny is a software implementation of the USB low-speed protocol for the Atmel ATtiny microcontrollers. Of course, it will also work on the ATmega series. The software is written for an AVR clocked at 12 MHz. At this frequency, each bit on the USB bus takes 8 clock cycles, and with a lot of trickery, it is possible to decode and encode the USB waveforms by software. The USB driver needs between 1300 and 1400 bytes of flash space (excluding the optional identification strings), depending on the configuration and compiler version, and 46 bytes RAM (excluding stack space). The C interface consists of 3 to 5 functions, depending on the configuration." - Link.

Related:

• USB into microcontroller: IgorPlug-USB (AVR) - Link.
• AVR USB - Link.
• AVR projects @ MAKE - Link.