A third to a half of articles have a free Open Access version, but finding them isn't always easy.
Save time with these easy-to-install Open Access tools that instantly search aggregated repositories, Open Access journals, preprint servers, etc.
CORE Discovery finds Open Access copies of articles, using the CORE dataset of millions of full text OA papers, as well as the data of other providers.
CORE Discovery says it has "the best grip on open content from open repositories as opposed to other services that disproportionately focus only on content indexed in major commercial databases." and that it is the only discovery system that can locate scientific content even for items with an unknown DOI or which do not have a DOI.
Another useful feature of CORE is that if the paper you're looking for isn't available Open Access then CORE will suggest other papers on the same topic that are Open Access. More details at: https://blog.core.ac.uk/2019/11/29/releasing-a-new-core-discovery-browser-extension/
Open Access Button searches thousands of sources containing millions of articles to connect you to an article instantly if it is freely available.
They say their sources include all of the aggregated repositories in the world, hybrid articles, open access journals, and those on authors personal pages.
The OA Button also provides a service for requesting an article from the author if you don't have access via subscription and the article isn't yet OA.
We previously wrote a blog post about Unpaywall: https://isnews.stir.ac.uk/2018/04/11/unpaywall-find-free-legal-open-access-articles/
Unpaywall searches the Unpaywall database which works by harvesting from Open Access sources to find OA content and then matching that content to the Crossref DOI, so for any DOI searched via the plugin, Unpaywall "knows about any OA versions that exist anywhere".