EBSCO Discovery Service indexes a large high-end full text collection of content as well as entire library collections. EBSCO’s resources are available in hundreds of thousands of institutions worldwide.
OCLC WorldCat is an online computer library containing high quality, global
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.
Semantic Scholar is a project developed at the Allen Institute for Artificial Intelligence. Publicly released in November 2015, it is designed to be an AI-backed search engine for scientific journal articles. Semantic Scholar is designed to highlight the most important and influential papers, and to identify the connections between them.