HAL (short for Hyper Articles en Ligne 'Hyper Articles Online') is an open archive where authors can deposit scholarly documents from all academic fields. 

Google Scholar is a freely accessible web search engine that indexes the full text or metadata of scholarly literature across a range of publication formats and disciplines. 

arXiv is an open-access repository of electronic preprints and postprints (known as e-prints) approved for posting after moderation but not peer-reviewed.

The Web of Science (WoS; previously known as Web of Knowledge) is a paid-access platform that provides (typically via the internet) access to multiple databases that provide reference and citation data from academic journals, conference proceedings, and other documents in various academic disciplines.