1. Google Академия 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. Google Scholar index includes most peer-reviewed online journals of Europe and America's largest scholarly publishers. This product provides the citation indexing features. While Google does not publish the size of Google Scholar's index, third-party researchers estimated it to contain roughly 160 million documents (link)

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

3. Socionet  is a data aggregator that forms a virtual scientific environment in the spirit of Open Science for creating and using information resources and services addressed to professional scientific communities (link)

4. Mendeley is a free reference manager and academic social network that can help you organize your research, collaborate with others online, and discover the latest research (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

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

7. 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)

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

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

10. 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)

11. 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)

12. Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content to everyone, in their own language (link)