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Unpaywall - an open database of free scholarly articles
Other Library databases where search results can be easily limited to AO content, e.g. Scopus
Scholar Google Author Profiles
The Public Access section of a Google Scholar profile contains the articles that are publicly available.
An article can be publicly available from several sources including its publisher, an institutional repository, a research area specific repository and others. The Google Scholar indexing system tries to include all publicly accessible versions that follow their inclusion guidelines.