DRMBuster - DRMBuster is a powerful all-in-one audio and video conversion software for any kind of purchased or legally obtained music and video.
OpenAudible - OpenAudible Project Page
Apprentice Alf's DeDRM Tools - Everything you ever wanted to know about DRM and ebooks, but were afraid to ask.
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.
AppleMacSoft DRM Converter for Mac - Legal Convert iTunes and Apple Music M4P to MP3 on Mac with DRM Converter 3.
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.