Academic Journals Database--"a universal index of periodical literature covering basic research from all fields of knowledge, and is particularly strong in the humanities and the social sciences."
BASE--is operated by the Bielefeld University Library. It "is one of the world's most voluminous search engines especially for academic open access web resources."
Directory of Open Access Books--"The primary aim of DOAB is to increase discoverability of Open Access books"
Directory of Open Access Journals "DOAJ has over 7,000 journals covering agriculture and food service, art and architecture, biology and life sciences, business, chemistry, health science, history, literature, mathematics and statistics, philosophy and religion, physics, social science, and engineering."
DSpace International--DSpace pioneered by MIT and Hewlett Packard is now used by over 800 repositories around the world including 131 academic libraries in the United States. You may search any repository listed on this link.
eScholarship--The University of California system publishes 45 open access journals available on this site.
MDPI Open Access "MDPI is a pioneer in scholarly open access publishing and has supported academic communities since 1996."
Networked Digital Library of Theses and Dissertations--an open access database with access to abstracts and full-text theses and dissertations from around the world.
OAIster-- union catalog of millions of records representing open access digital resources.
Open Thesis--"is a free repository of theses, dissertations, and other academic documents, coupled with powerful search, organization, and collaboration tools."
PQDT Open: "read the full text of open access dissertations and theses"
Project Gutenberg: "offers over 40,000 free ebooks: choose among free epub books, free kindle books, download them or read them online."
Science Direct: Elsevier publishes 56 open access journals.