http://www.base-search.net/about/en/

The Open Access Academic Resources Register is operated by the Universitätsbibliothek Universität Bielefeld. It allows searching the content of the repositories through a simple and advanced search interface. One interesting way of searching is the possibility to use the multilingual thesaurus EUROVOC (allowing search terms to be translated into language equivalents in several European languages). Searches can be limited not only in time but also in location (per country or continent). You can search by document type or by the Dewey Decimal Classification.

https://core.ac.uk/about

CORE is an aggregator of several million publications from hundreds of repositories around the world, which creates a number of added services and user applications on top of the data. For the average user, the most interesting part of CORE is probably the portal, which allows searching through publications. It allows basic and advanced searches. Records can be filtered by language, year of publication, authors and source repository.

http://ezb.uni-regensburg.de/fl.phtml?bibid=AAAAA&colors=1&lang=en&notation=all

A selection of freely available titles within the EZB database. More than 20,000 freely available journals and electronic documents are included in the electronic library.

http://www.doabooks.org/

The Open Books Register includes mostly books published by academic publishers. The register allows searching and browsing of book records. The records contain basic descriptive information about the books, such as author names, the title, ISBN, publisher, language, basic content classification, as well as keywords and abstract, a link to the full text, a link to where to buy a hard copy of the book, the ability to export a book citation, and the ability to share a link to the book. The Browse by Publisher section also provides basic information about publishers and the peer review process.

http://www.doaj.org

The open access journal registry includes records of peer-reviewed open access journals. The registry allows searching and browsing of journal records, as well as searching the article records of approximately half of the registered journals. The records contain basic descriptive information about the journals, such as the title, ISSN, publisher, language, basic content classification, as well as keywords and information about royalties for publication in the journal.

https://figshare.com/

A general repository for all types of publication results in any format.

http://www.americangeosciences.org/georef/open-access-journalsseries

A list of freely available Earth science journals.

http://oaister.worldcat.org/

It searches freely available and open content.

https://www.openaire.eu/search/find

A registry set up for the OpenAIRE project. It includes publications from linked repositories created under FP7 funded projects. It allows searching by document type, year of publication, source repository, project designation, etc.

http://www.opendoar.org/

The Open Access Digital Library Register is run by the University of Nottingham in the UK. It includes records of institutional and subject repositories and other types of digital libraries from around the world. It allows easy searching and browsing by several criteria such as country (continent), type of digital library, content, document type, language of records, etc.

https://open-research-europe.ec.europa.eu

The European Commission's open multidisciplinary publishing platform for Horizon Europe publications.

http://paperity.org/

Aggregator of articles from OA journals.

https://protocolexchange.researchsquare.com/

Repository for sharing experimental workflows and protocols.

https://www.protocols.io/

Data management and protocol sharing platform.

https://www.re3data.org/

Data repository aggregator.

http://roar.eprints.org/

The Registry of Open Access Repositories is operated by the School of Electronics and Computer Science at the University of Southampton, USA. It allows searching and browsing of institutional records, subject repositories and other digital libraries. Each record includes information on availability, the number of records, content and time coverage, etc.

http://road.issn.org/

The ISSN Registry of the UNESCO International Centre for Communication and Information Sector Support provides free access to scholarly OA resources that have been assigned an ISSN. The main objective of the registry is to provide uniform access to different types of online scholarly resources published worldwide and to inform about their quality.

https://www.zenodo.org/

A general aggregator for data, software and repositories.

https://osf.io/registries

https://aspredicted.org/

https://preclinicaltrials.eu/

https://www.crd.york.ac.uk/prospero/

https://egap.org/

https://www.3ieimpact.org/evidence-hub/ridie

https://www.preprints.org/

 A multidisciplinary register of preprints of scientific outputs.

https://www.biorxiv.org/

 A disciplinary register of preprints of scientific articles.

https://www.medrxiv.org/

A disciplinary register of preprints of scientific articles.

https://psyarxiv.com/

A disciplinary register of preprints of scientific articles.

https://osf.io/preprints/socarxiv

A disciplinary register of preprints of scientific articles.

http://lawarxiv.info/

A disciplinary register of preprints of scientific articles.

https://arxiv.org/

A disciplinary register of preprints of scientific articles.

https://github.com/

A development platform for hosting and reviewing code, managing projects and creating software.

https://savannah.gnu.org/

A registry of free projects that run on free operating systems, with a focus on GNU software.

https://sourceforge.net/

An open source hosting platform.

https://launchpad.net/

A platform for collaboration and hosting software.

https://dspace.zcu.cz/

The Digital Library of the University of West Bohemia is an institutional repository of the University of West Bohemia, whose primary goal is to store and provide unlimited online access to the full texts of publications by authors from the University of West Bohemia, journals and proceedings published at the University of West Bohemia and qualification theses defended at the University of West Bohemia. The repository is accessible in English and Czech and contains contributions in English, Czech and Slovak.

https://www.dspace.cz/dspace-v-cr

A directory of digital libraries of Czech universities with the DSpace system.

http://dml.cz/

The discipline-specific digital library operated by the Institute of Mathematics of the Academy of Sciences of the Czech Republic includes full texts of articles, conference papers, monographs, etc. from the field of mathematics published in the Czech Republic.

http://theses.cz

The Czech National Register of Qualification Theses is used to detect plagiarism and to search and retrieve freely available full texts of qualification theses defended at selected Czech and Slovak universities and colleges involved in the eponymous project.

http://invenio.nusl.cz/

This national digital library includes over 100,000 full texts of information sources falling into the category of so-called grey literature, mainly in technical fields. It contains mainly qualification theses, contributions from proceedings, research reports, annual reports, etc.

http://trove.nla.gov.au/book/result?q&l-format=Thesis

The sub-base of the Australia Trove digital library, operated by the National Library of Australia, provides free access to the full texts of qualification theses (mostly dissertations) defended at Australian universities and colleges.

http://dissonline.de

A multidisciplinary digital library of dissertations and habilitation theses, which is being created by the Deutsche Nationalbibliothek. It contains over 80,000 freely available full texts of dissertations and habilitation theses defended at German and Swiss universities and colleges. Most of the included theses are in German only.

http://scholar.lib.vt.edu/theses/

Virginia Polytechnic Institute's digital library of qualification theses includes more than 18,000 full-text theses and dissertations in architecture, engineering, economics, agriculture, science, and more.

http://www.ndltd.org/

A shared digital library of qualification theses. The system is maintained by a collaboration of 76 institutional and three consortium members from around the world. It contains about 190,000 qualification theses in different world languages (mostly in English).

http://tel.archives-ouvertes.fr/

A multidisciplinary digital library of dissertations operated by the French Centre pour la Communication Scientifique Directe. It contains over 17,000 full-text dissertations, mostly in French.

http://memory.loc.gov/ammem/index.html

The multimedia digital library managed by the Library of Congress of the USA includes thematically arranged collections of primary documents - texts, images, maps, audio and audiovisual recordings, etc., related to the history of the North American continent, especially the history of the USA.

http://arxiv.org

A disciplinary repository for mathematics, physics, technology and engineering is maintained by Cornell University and includes over 700,000 e-prints of articles, research reports, etc.

http://www.biomedcentral.com/

The digital library of the eponymous publisher, includes the full text of articles from 220 journals in the field of biology and biomedicine.

http://cdsweb.cern.ch/

Institutional repository operated by the European Organization for Nuclear Research, CERN.

http://cogprints.org/

The digital library operated by the University of Southampton includes e-prints of articles, conference papers, qualification theses in psychology, linguistics, philosophy, as well as related documents in neuroscience, computer science, biology, etc. It includes over 3 thousand documents.

http://www.econstor.eu/

The digital library operated by the Deutschen Zentralbibliothek für Wirtschaftswissenschaften and the Leibniz-Informationszentrum Wirtschaft includes mainly research reports, e-prints of articles, conference papers, qualification theses, books and complete volumes of selected journals. It includes documents in the field of economics in English, German, Russian, etc.

http://www.eric.ed.gov/

This partially full-text database, funded by the U.S. Department of Education, contains documents primarily in the field of education (and partially in related fields such as school and college librarianship). It includes various types of documents - articles, research reports, books, teaching materials, computer programs, qualifying papers, etc.

http://www.egms.de/dynamic/en/index.htm

A digital library operated by the Association of Scientific Medical Societies in Germany in cooperation with the Deutsches Institut für Medizinische Dokumentation und Information, DIMDI and the Deutsche Zentralbibliothek für Medizin. It includes mainly e-prints of articles and conference papers in the field of biomedicine and related sciences. Includes documents in English and German.

http://ieeexplore.ieee.org/Xplore

A digital library containing 3,000,000 freely available documents in the field of engineering and technology.

http://www.osti.gov/collections.html

The digital library established by the U.S. Department of Energy includes e-prints of articles, patents, conference papers, research reports, software applications, etc. in the field of energy and engineering in general.

http://philpapers.org/

A distributed digital library operated by the British Institute of Philosophy, University of London. It includes documents from various philosophical disciplines such as philosophy of science, logic, ethics aesthetics, history of philosophy, etc.

http://www.ncbi.nlm.nih.gov/pmc/

This partially full-text database maintained by the U.S. National Institutes of Health provides access to e-prints of articles, conference papers, reports, etc. It includes retrospective documents from 1990 onwards in the fields of biology, biomedicine, biochemistry, etc.

http://repec.org

RePec is a distributed digital archive focusing on economic disciplines. It includes e-prints (preprints and postprints) of journal articles, research reports, books, individual chapters, software components, etc.

http://www.ssoar.info/

The multidisciplinary digital library, operated by the Leibniz Institut für Sozialwissenschaften and the Freie Universität Berlin, contains e-prints of articles, conference papers, books, qualifying theses, research reports, etc. from the social sciences - law, philosophy, history, philology, theology, sociology, psychology, information science, political science, economics, ethnography, as well as documents from borderline and related fields.

http://vads.ac.uk/

The digital library run by the University of the Creative Arts London contains photographs of artworks (paintings, sculptures, applied art) divided into thematic collections.

https://www.qoam.eu/

A database of open access journals that assigns scores to relevant journals based on author ratings and experience.

http://jane.biosemantics.org/

A tool that compares your article abstract with the MEDLINE database and recommends appropriate journals for the topic. The disadvantage of JANE is its narrow subject focus; it will be used primarily by authors who publish in fields such as medicine, nursing, biology or biochemistry.

https://thinkchecksubmit.org/

A set of questions to help you assess whether it is a trustworthy journal and you can publish in it without any concerns. 

http://citeseer.ist.psu.edu

A citation database for open access resources, maintained by Pennsylvania State University, indexes information resources primarily in the fields of information and computer science. It allows you to track citation links and view a list of references. In case of direct availability of a cited document, it allows immediate access in the form of a link.