Another World is Possible - Small library of anarchist audiobooks on Archive.org
Free Radical Radio Audiobooks - Audiozines and snippets of Audiobooks
Leftist Theory Audiobooks (YT) - Leftist texts hosted on Youtube
Loyal books - 7000+ audiobooks
OpenCulture - 900 audiobooks
Lit2Go - Literature classics from a university library
French: http://www.litteratureaudio.com/ - 6000+ audiobooks
Piracy (Use privacy software/VPN)
Audiobook Bay - huge selection
Unwelcome Guests Audiobooks - a few direct MP3 downloads
Android: Voice (Audiobook Player) - Great interface and features, and of course free/GPL software.
iOS: Audiobook-Player/BookPlayer - free/GPL software
If you know additional sources/software to list, feel free to edit this yourself or post in /f/audiobooks.