More concrete uses:
- USB protocol analyser driver in userland (the protocol analyser is itself an
  USB device). Rather simple code, uses synchronous and asynchronous bulk
  transfers.
    https://github.com/vpelletier/ITI1480A-linux/blob/master/iti1480a/capture.py
- Tinkerforge's brickd
  They since switched to all-C to fit into embedded devices.
    https://github.com/Tinkerforge/brickd/tree/v1.x.y/src/brickd
- NetMDPython
  Tools to control NetMD Mindisk players by USB.
    https://wiki.physik.fu-berlin.de/linux-minidisc/doku.php?id=netmdpython
