If you have an article and are looking for an open access version of it, try one of the options below. Neither of these services has 100% coverage of OA scholarship, but both are easy to use and provide access to a high percentage of materials in institutional repositories and on preprint servers.
Open Access Button - Enter an article's URL, DOI, citation, title, PMC ID, or PubMed ID to search for an OA version of the article - You can also search for datasets with the Open Access Button - A browser extension is also available - click on "Get the browser extension" link from the Open Access Button's homepage - If your article is not found, Open Access Button will give you an option to request the article from the author
Unpaywall - A browser extension for Chrome and Firefox that searches for open access versions of an articles
In 2019, the Washington Resarch Library Consortium (WRLC), of which Georgetown University is a member, joined the Open Textbook Network to support and promote the use of open textbooks.
Federal Agencies (info from the University of Minnesota Libraries) Policies vary somewhat from agency to agency; many allow for a 12-month embargo period before being made publicly available in an open access repository.