DOAJ is an independent community-curated online directory that indexes and provides access to high quality, open access, peer-reviewed journals.

http://doaj.org

Elsevier Open Access Journals about science have undergone peer-review, immediately free to access and download from ScienceDirect, and permitted to reuse defined by the author’s choice of Creative Commons user licenses.

https://www.elsevier.com/about/open-science/open-access/open-access-journals

Frontiers provide open access online journals led and peer-reviewed by top researchers. It covers more than 600 academic disciplines.

https://www.frontiersin.org/

Google Scholar is a web search engine that indexes full-text and metadata scholarly literature across various publishing formats and disciplines.

https://scholar.google.com/

Scientific Research Publishing (SCIRP) is one of the largest Open Access journal publishers. It is currently publishing more than 200 open access, online, peer-reviewed journals covering a wide range of academic disciplines. SCIRP serves the worldwide academic communities and contributes to the progress and application of science with its publication.

https://www.scirp.org/index.aspx