ATR-42 300 has two wing mounted APUs that even provide some forward thrust through attached propellers. 500 series has real engines,though.
Now seriously: prop brake seems to be ATR's specialty. Usually it's installed on No2 eng only to use HP spool as APU but not quite. Engine intake is to close to exhaust and even 2kt of tailwind component provides your unlucky pax with some burnt kerosene aroma. Also exhaust is directed at service door, making baggage loading difficult at the very least.