This video by Ray Wang demonstrates HID-class USB serial communication for AVRs using the V-USB library:
Serial communication is helpful for debugging and checking sensor values, and HID-class means it can work on all operating systems without driver installation. This is useful for many software-only USB projects such as based on ATmega328 or ATtiny45/85. To receive and send messages between the device and host, I wrote an HID serial monitor, similar to a standard serial monitor, using Processing.
You can find his post at RaysHobby.
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Nice find