I mainly use my Sansa Zip Clip but if the battery goes dead I have a Samsung Galaxy Player 4.2 as a back up if it's MP3 or Audible. If it's WMA then I have a Sansa Clip+ that I can use while the Zip charges.
For MP3 Audiobooks when using Android I use the
Smart AudioBook Player by Alex Kravchenko which is an amazing app. I bought the full version.