Raph has designed this board for NES/SNES controllers to USB, but it found place in many more projects due to its small size and easy to use.
This board is only 27.5×17.7mm size and is equipped with Atmega8 microcontroller. Circuit is already adapted for Firmware-Only USB Driver from Objective development including 12MHz resonator. Also there are several I/O pins available for custom programming. Because board it self is double sided, there is a place for SMD voltage regulator.
Multipurpose tiny AVR microcontroller board - [Link]