Some time ago someone from the lab found userial - a USB to I2C/SPI/GPIO/ADC bridge (http://www.tty1.net/userial/). It seemed like an useful toy - USB to I2C and couple of IO's to play with. Basically connect it to your computer, fire up your HTerm and you can speak in I2C. But the cost of the chip used was enormous, so I changed the firmware to accept readily available atmega16u4. Couple of IO's and ADCs were lost during the process, but nothing really bad. The new firmware is here http://jaanus.tech-thing.org/img/userial-firmware-u4.tar.bz2
The hardware we use is atmega16U4 development board what you can find from Erik's page http://pro.a5d.biz/atmegaxxu4-development-board/ But it works with whatever hardware, so you can use it to test if your design works, without writing single line of code.
It was and will be again released under MIT license, it permits to share and modify it as you please.