OpenAudible - OpenAudible Project Page
LibriVox - free public domain audiobooks
DRmare Audio Converter - DRmare Audio Converter for Windows is a complete audio DRM removal tool designed to convert both protected audios, including Apple Music, Audible audiobooks, iTunes music, and common audios to MP3, WAC, FLAC, M4A, AAC, etc.
DigitalBook.io - Free audio books and eBooks. Search, browse, listen, read and download over 100,000 free audio books and eBooks.
TunesKit Audible AA/AAX - Audible AA/AAX Converter by TunesKit is an application that enables you to convert audible AA and AAX files to the formats that are supported by other devices.
Downpour.com - Listen to high quality audiobooks instantly. Download audiobooks to your iPhone, Android, or other listening device. Find titles including best-sellers and new releases. Downpour, where content reigns.