Bielefeld Academic Search Engine (BASE) is 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 Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH). BASE provides more than 70 million documents from more than 3,000 sources. Where available, bibliographic data is provided, and the results may be sorted by multiple fields, such as by author, Dewey Decimal Classification or year of publication.

Google Scholar 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.

OpenAIRE (Open Access Infrastructure for Research in Europe) is a project that supports the European Commission Open Access policy by providing an infrastructure for researchers to comply with the European Union Open Access mandate. This repository provides content across academic disciplines with over 11,5 million scientific publications, found in journal articles, dissertations, books, lectures, reports, etc., harvested regularly from more than 610 repositories, from 52 countries.

WorldCat is a global union catalog and the world's largest bibliographic database that itemizes the collections of 72,000 libraries in 170 countries that participate in the Online Computer Library Center (OCLC) global cooperative. It contains more than 330 million records, representing over 2 billion physical and digital assets (articles, archival materials, books, journals, maps, music, videos, and other resources) in 485 languages.

Scientific Periodicals of Ukraine (SPU VNLU) Producer: Vernadsky National Library of Ukraine Scientific Periodicals of Ukraine - project Vernadsky National Library of Ukraine (VNLU). Ukrainian scientific periodicals contain articles from international journals in all fields of knowledge and carries out the completion of full-text electronic information resources, electronic counterparts publications or electronic publications provided by the owners. As part of an information resource "Scientific Periodicals of Ukraine" is adjusted full-text search of scientific publications in PDF format. VNLU spent working on the integration of information resource "Scientific Periodicals of Ukraine" to the interface of analytical services, Google Scholar (Google Academy). It contains approximately 2520 - journals; 25372 - issues; 628524 - full texts of articles.