Simple USB I²C adapter using an ATTiny2313A and the VUSB library.
Supports most of the standard I²C commands (i2cdetect, i2cget, i2cset) and can handle I²C both in soft- and hardware, thus allowing debugging / protocol adjustments without updating the device firmware. The SMD version is only slightly bigger than a USB-TTL adapter.
Proper documentation is still a TODO. In the meantime, see vusb-i2c on GitHub for source code.
Note that this project also serves as a playground for PCB design und VUSB usage. There are already more mature projects for interfacing USB to I²C and other protocols, such as the littlewire project.