There can be several variations of quality amongst these services, so we can categorize them or give the services some score based on that. For example:
1. [good]: service doesn't proliferate DRM - it doesn't accept content with DRM from the publishers and offers strictly DRM free content.
2. [mediocre]: Service offers DRM free content, but offers DRMed as well. It doesn't prevent proliferation, but it clearly warns its users about the presence of DRM.
3. [bad]: Service offers DRM free and DRMed content and doesn't warn about the presence of DRM - users have to deduce / research each title.
4. [very bad]: The content has DRM but it's is obsolete (i.e. trivially breakable, removable).
I'm not sure about each service, so comments and proposals are welcome. Note that music is not included, since the music industry got rid of DRM for the most part already. The list is work in progress.
- Internet Archive - texts [good] [free]
- Project Gutenberg [good] [free]
- Project Gutenberg Australia [good] [free]
- The Book Depository free ebooks [good] [free]
- Tor ebooks [good]
- O'Reilly [good]
- Manning Publications [good]
- Cory Doctorow's books [good] [pay if you want]
- Arxiv - open access library for scientific articles [good] [free]
- GOG [good] [DRMed OS versions]
- Humble Bundle [mediocre] [DRM-free OS versions]
- Desura [mediocre] [DRM-free OS versions]