1. SCOPUS is the largest abstract and citation database of peer-reviewed literature: scientific journals, books and conference proceedings (link)

2. Google Аcademy is a freely accessible web search engine that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines (link

3. Mendeley is a unique solution that combines a reference manager (a means of collecting and cataloging scientific articles for the subsequent preparation of whiteboard lists) and a scientific social network, which allows you to organize a personal scientific library, work together on articles, as well as find like-minded people and explore trends in modern research (link)

4. Neliti is a research repository that helps researchers and decision makers in Indonesia find research, data and evidence (link)

5. OpenAIRE is the European Research Information system that encompasses all research and enables ready-made reporting, monitoring and analysis. OpenAIRE link research outcomes (e.g., publications, data, software) to their creators (e.g., researchers, institutions, funders), enabling discoverability, transparency, reproducibility and quality-assurance of research. (link)

6. Zenodo is a general-purpose open-access repository developed under the European OpenAIRE program and operated by CERN. Each author has the ability to track the number of views and downloads of their publications that are in the repository. (link)

7. Dimensions is a dynamic, easy to use, linked-research data platform that re-imagines the way research can be discovered, accessed and analyzed (link)

8. BASE (Bielefeld Academic Search Engine) one of the world's most voluminous multi-disciplinary search engines especially for academic open access web resources. This engine collects, normalises, and indexes data using the OAI-PMH) (link)

9. CORE (COnnecting REpositories) is aggregate all open access content distributed across different systems, such as repositories and open access journals, enrich this content using text mining and data mining, and provide free access to it through a set of services (link)

10. The Internet Archive is an American digital library with the stated mission of "universal access to all knowledge (link)

11. Digar is a digital archive of the National Library of Estonia, or digital library where you can find books, newspapers, journals, maps, notes, photographs, postcards, posters, illustrations, audio files and music files (link)

12. ESTER is the shared catalogue of the largest libraries of Estonia. The catalogue also contains records of items stored in university colleges and special libraries (link)

13. Scilit is a comprehensive, free database for scientists using a new method to collate data and indexing scientific material. This database of scholarly works is developed and maintained by the open access publisher MDPI (link)

14. Figshare is an online scientific repository where users can make their research results available (link)

15. The Open Ukrainian Citation Index (OUCI) is a search engine and a citation database that come from all publishers that use the Crossref’s Cited-by service and supports the Initiative for Open Citations (link)

16. The Estonian national bibliography database ERB registers data on national documents. Publications published in Estonia in all languages ​​and abroad, works of Estonian authors and their translations regardless of their physical medium (paper, electronic medium) are considered national documents (link)

17. ENGINEERING RESEARCH PAPERS (Engpaper) the repository of scientific content in technical disciplines, with over a million articles available (link)

18. Semantic Scholar - designed to be an AI-backed search engine for academic publications (link)

19. The Lens serves almost all the patents and scholarly work in the world as a free, open and secure digital public good, with user privacy a paramount focus (link)

20. EcobBiz is subject portal for economics and business studies. It is provided by the German National Library of Economics – Leibniz Information Centre for Economics (ZBW) (link)