Emulate USB device with USB host
I found the Facedancer board, which looks interesting. It contains a FTDI, a microcontroler, and a MAX3421E USB controler, and allows you to emulate a USB device in Python! That is, you basically write bit-banging code in Python on your PC, and the other PC sees the USB device that you emulate. It could be interesting for prototyping, and it has been used for fuzzing (automatically finding errors in USB host implementations and software).