JSTOR Open and Free Content: Browse open and free content made available through JSTOR and Artstor.
https://about.jstor.org/oa-and-free/

Academic Journals: Academic Journals' mission is to accelerate the dissemination of knowledge through the publication of high quality research articles using the open access model.
https://academicjournals.org/

Google Scholar: Provides a search of scholarly literature.
https://scholar.google.co.in/

Paperity: Paperity is a multidisciplinary aggregator of Open Access journals and papers.
https://paperity.org/

Internet Archive: A non-profit digital library offering free universal access to books, movies & music, as well as archived web pages. In this digital library, two million free texts are now available.
http://archive.org/