Google Scholar
Google Scholar provides a simple way to broadly search for scholarly literature. From one place, you can search across many disciplines and sources: articles, theses, books, abstracts and court opinions, from academic publishers, professional societies, online repositories, universities and other web sites. Google Scholar helps you find relevant work across the world of scholarly research.
URL: https://scholar.google.com/

CrossRef

Crossref makes research objects easy to find, cite, link, assess, and reuse. We’re a not-for-profit membership organization that exists to make scholarly communications better.

URL: https://search.crossref.org/

ResearchBib

ResearchBib is open access with high standard indexing database for researchers and publishers. Research Bible may freely index journals, research papers, call for papers, research position.

URL: https://www.researchbib.com/

WorldCat

WorldCat is the world's largest network of library content and services. WorldCat libraries are dedicated to providing access to their resources on the Web, where most people start their search for information.

URL: https://www.worldcat.org/

Citefactor
Citefactor is a service that provides access to quality controlled Open Access Journals. The Directory indexing of journal aims to be comprehensive and cover all open access scientific and scholarly journals that use an appropriate quality control system, and it will not be limited to particular languages or subject areas. The aim of the Directory is to increase the visibility and ease of use of open access scientific and scholarly journals thereby promoting their increased usage and impact.
URL: https://www.citefactor.org/