Anything related to printer controllers not being Duet, RAMPS or Sanguinololu: microprocessors, motor and heater driver circuitry, gEDA/KiCAD issues, soldering tips, etc.
We have 10 Teensylu -0.7 boards here at Hackerspace Charlotte, one has been tested through stepper motion. We may print with it soon.
6 others are being built as a group activity.