- Google Scholar
- The Online Books Page
- Gallica – Digital library of the Bibliotheque national de France.
- Internet Archive – A non-profit library of millions of free books, movies, software, music, and more.
- KVK – Karlsruher Virtuelle Katalog
- WorldCat– The World’s Largest Library Catalog
- World Digital Library
- Europeana
- Gutenberg Projekt
- HathiTrust Digital Library
- OAPEN – Open Access Publishing in European Networks
- DOAJ – Directory of Open Access Journals
- DOAB – Directory of Open Access Books
- Open Textbook Library
- ROAR-Registry of Open Access Repositories
- DART Europe E-Theses Portal
- Hungarian Repository Common Search
