Directory of Open Access Books (DOAB): The primary aim of DOAB is to increase discoverability of Open Access books. The directory will be open to all publishers who publish academic, peer reviewed books in Open Access and should contain as many books as possible, provided that these publications are in Open Access and meet academic standards.

Directory of Open Access Journals (DOAJ): DOAJ is an online directory that indexes and provides access to quality open access, peer-reviewed journals.

OAPEN Library: OAPEN provides a platform for the full text dissemination of Open Access books. OAPEN builds a quality controlled collection of Open Access books, mainly in the area of Humanities and Social Sciences, and develops services for publishers, libraries and research funders in the areas of dissemination, quality assurance and digital preservation.

Bielefeld Academic Search Engine (BASE): BASE is one of the world's most voluminous search engines especially for academic open access web resources and is operated by Bielefeld University Library.
A important feature of BASE is to discloses web resources of the "Deep Web", which are ignored by commercial search engines or get lost in the vast quantity of hits.

Open Access Button : Open Access Button makes finding freely available versions of scientific publications that were originally fee-based possible. If a free version of the desired text is not available, Open Access Button contacts the author of the publication so that it can be made accessible to everyone. The search can be carried out via a web interface or using the available browser extension.

Unpaywall: Unpaywall is a browser plugin that allows you to find freely available versions of a closed-access document you have just consulted. This makes it easy for example to find texts that have been published as secondary publications.

CORE Discovery: CORE Discovery is another browser extension that allows you to find free versions of fee-based articles. It can be used with the browsers Chrome, Firefox and Opera.

EconStor: EconStor is the Open Access server of the German National Library of Economics – Leibniz Information Centre for Economics. EconStor provides a basis for the free publication of academic literature in economics.

RePEc: RePEc (Research Papers in Economics) is a collaborative effort of hundreds of volunteers in 96 countries to enhance the dissemination of research in Economics and related sciences. The heart of the project is a decentralized bibliographic database of working papers, journal articles, books, books chapters and software components, all maintained by volunteers. The collected data are then used in various services that serve the collected metadata to users or enhance it.

arXiv: arXiv is a freely accessible archive for electronic preprints and postprints. It contains scientific articles from a wide range of scientific disciplines such as mathematics, physics, astronomy, electrical engineering, computer science, quantitative biology, statistics, financial mathematics and economics.

engrXiv: engrXiv (Engineering Archive) is a non-profit archive for engineering preprints that has been in operation since 2016.

ChemRxiv: ChemRxiv is an open access preprint archive for the field of chemistry, which has been active since 2017 and is operated by the American Chemical Society, the Royal Society of Chemistry and the German Chemical Society.