Sites and apps similar to OAPEN include Internet Archive, which is free. Other options are arXiv and PaperLens. In total there are three Ebook Libraries like OAPEN.
The Internet Archive is building a digital library of Internet sites and other cultural artifacts in digital form. Like a paper library, it provides free access to researchers, historians, scholars, the print disabled, and the general public.
United States
arXiv® is a free distribution service and an open archive for scholarly articles in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics.
United States
