Several devices. My very favorite is my
Sansa Clip Zip. It's cheap, small, and has native support for the major audiobook formats (MP3, Audible, protected WMA). Library audiobooks are one of my major sources and the majority of their catalogs are in protected WMA (over 99%), with MP3 trailing far behind (67-75%). Android devices don't support protected WMA and it's hit or miss with iOS devices (up to the publisher). Format shifting is an emphatic no go, because these books are borrowed.
If it's Audible and I'm at home, I'll generally use my Fire HD 7. Traveling, I'll use whatever I have with me - Clip Zip, Nexus 4, or Nexus 7.