Dimensions: Open science
Aggregates millions of open science publications, grants, policy documents, citations, and more. Covers the health, chemical, physical, and clinical sciences, as well as engineering, mathematics, psychology, and much more.

DOAJ Directory of Open Access Journals (OA)
Free, full-text scientific and scholarly journals of all subjects and languages.

Google Scholar
While many articles are behind institutional subscription paywalls, you can often find links to open access archives for scholarly materials.

JSTOR Open Content
Academic content on JSTOR that is open to everyone, everywhere. Search thousands of free journal articles and open access book chapters. JSTOR also offers free individual accounts to their full scholarly collection, allowing up to six article downloads every 30 days.

NCBI Bookshelf
Free online access to ebooks and documents in life sciences and healthcare.

Project Muse OA
Open access (OA) books and journals from several distinguished university presses and scholarly societies.

Public Library of Science (PLoS)
PLoS publishes peer-reviewed, open access scientific and medical journals.

PubMed Central
PubMed Central is a free digital repository that archives publicly accessible full-text scholarly articles that have been published within the biomedical and life sciences journal literature.