STEM - Free & Open Access Resources

If you prefer to start your for articles at a particular publisher's site, some will allow you to limit your searches to free/OA articles. Even if the publisher doesn't allow you to limit results by OA content, such as with Science Direct, the articles that are OA should be clearly identified in the results.

Publisher	How to Search for or Limit to OA Articles
American Association for the Advancement of Science	Can browse results for OA articles the journals (including Science Advances, which is fully OA), but no way to limit or refine a search to do, though you can limit to just Science Advances.
American Chemical Society	From the search results page, go to Refine Search, and under Access Type select Open Access Content, which will also include articles that were made freely available by the journal editors.
American Institute of Aeronautics and Astronautics	Limit to Open Access materials in the Advanced Search, or when you Refine Results.
American Institute of Physics	From the search results page, go to Refine Search and then Advanced. Select Open Access Content Only.
American Astronautical Society	From the search results page, check the box for Open Access Only. Also on the IOPscience platform.
American Physical Society	On the search page, check the Category box for Open Access. Or select Open Access from the Category list on the search results page.
American Society of Mechanical Engineers	From the search results, scroll down the "Update Search" column for Availability. Select Open Access and Public Access.
Association for Computing Machinery	Can browse results for OA and public access papers, but no way to limit or refine a search to do so.
BioMed Central	Gold OA journals, so every article is free to read.
Cambridge University Press	From the search results page, under Access select Only Show Open Access.
Cell Press	From the search results page, select the Open Access filter.
Electrochemical Society	From the search results page, check the box for Open Access Only. Also on the IOPscience platform.
European Mathematical Society	Many of the journals are OA, but there's no OA filter for the search results.
IEEE	From the search results page, select Open Access Only.
Institute of Physics	From the search results page, check the box for Open Access Only.
JSTOR (Open and Free Content)	Search covers free/OA content in JSTOR only.  (Full text coverage of journals from university presses, mostly social sciences and humanities, but some science journals covered. Most recent 3-5 years may not be available)
Nature Journals	Can browse results for OA articles the journals (including fully OA journals like Scientific Reports and Nature Communi9cations),
Oxford University Press	From the Advanced Search page or when editing the Search Results, under Availability check the boxes for Free and Open Access.
Proceedings of the National Academy of Sciences	From the Advanced Search page or when editing the Search Results, under Limit Results select Open Access Articles. Articles are freely available after 6 months, regardless of whether they were published OA or not.
Public Library of Science (PLoS)	Gold OA journals, so every article is free to read.
Royal Society of Chemistry	From the search results page, select Open Access under the Article Access filter.
ScienceDirect (Elsevier)	No way to filter or limit OA articles, but they are clearly identified in the search results.
Society of Industrial and Applied Mathematics	Free and OA articles are identified by "Full Access" in the results.
Springer	No way to filter or limit OA articles, but they are clearly identified in the search results.
Wiley Online Library	From the search results page, go to Access Status and select Open Access Content.