OASIS is maintained by SUNY. You can use advanced search to find textbooks.
JURN Returns a good number of Academic.edu results, which you have to be member of to access.
Mason OER Metafinder (MOM) Maintained by George Mason University, can be searched for more than books.
Open Research Library Beta version now available. Final version available October 2019 with the goal of providing access to 20,000 open access books. Must create a free account.
Google Advanced Search allows searching by Usage Rights. To find open access materials use filters, "free to use or share," "free to use or share even commercially," "free to use, share, or modify," and "free to use, share, or modify, even commercially."
JURN: Guide to Academic Search Lists a number of open access repositories, many of which are listed here.
GRAFT Beta version of JURN's cross-repository search. Claims to search all repositories.
Global Open Access Portal This is the Access by Region page of this United Nations supported program. By clicking on a specific nation one can see major open access resources in that nation.
Open Access Button Searches across repositories for open access scholarship.
Unpaywall Find open access versions of papers and other scholarly input.
SHARE Maintained or funded by the Association of Research Libraries, Center for Open Science, Institute of Museum and Library Services, and the Alfred P. Sloan Foundation.OASIS (Openly Available Sources Integrated Search) Created at SUNY/Geneseo. It is from vetted sources.
OpenAIRE An EU network of open access repositories, archives and journals.
OAIster Maintained by OCLA (WorldCat), this site describes itself as a union catalog of millions of records that represent open access resources. This link brings you to the description page. To search use Searching OAIster Only Records.
Digital Commons Network Brings together articles, book chapters, and other materials from hundreds of libraries and universities worldwide.
Paperity International aggregator. Picks up gold open access and articles appearing in "hybrid" journals.
CORE Searches open access repositories and journals. Comes out of the UK.
CORE! The Open Access Repository for the Humanities Maintained by the Modern Language Association (MLA). It allows you to search and upload material.
OpenDOAR (The Directory of Open Access Repositories) Maintained by the Centre for Research Communication. Another UK site. It also maintains a list of institutional repositories.
Europeana An aggregator of books, and also paintings, music, radio stations, and fine art, from European galleries, museums and libraries. Often slow and there are broken links, which is to be expected from such a large aggregator. Nonetheless useful.
SSRN Articles and preprints. Range of disciplines in the social sciences.
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