VUSB-I2C is a USB to I²C adapter using an ATTiny2313A and the VUSB library.
It 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.
Note that this project mostly exists as a playground for PCB design and VUSB. There are several more mature and feature-rich projects, such as littlewire.