I2CDriver by Excamera Labs

I²CDriver is an easy-to-use, open source tool for controlling I²C devices and a great tool to help with quick driver development and debugging. It works with Windows, Mac, and Linux, and has a built-in color screen that shows a live “dashboard” of all the I²C activity. It uses a standard FTDI USB serial chip to talk to the PC, so no special drivers need to be installed. The board includes a separate 3.3 V supply with voltage and current monitoring. It's kinda like a Bus Pirate with a display and great Python support.