The Internet Archive is a non-profit digital library offering free access to websites, books, audio, videos, and more. Its mission is to provide Universal Access to All Knowledge for researchers, educators, and the public.
Project Gutenberg is the world’s oldest digital library. Started in 1971, it offers over 75,000 free eBooks, mostly classic books in the public domain. It's run by volunteers and aims to make reading easy and free for everyone around the world.
Springer Open journals and books are free to read online as soon as they are published. They go through expert review to make sure the research is high-quality and reliable.