BioMedCentral is a UK-based publisher of over 200 open access journals of biology. 

ChemistryCentral, like BioMedCentral, is an open access publisher of over 15 journals of chemistry; it is part of Springer publishing company.

Citizen Science Quarterly...?

Google Scholar is often able to find PDFs that have been uploaded to the author's own website etc. 

International Journal of Biological Sciences is an open access scientific journal published by Ivyspring International. 

Open Biology is the Royal Society's open access journal for molecular and cell biology.

Public Library of Science publishes seven open access journals.

Royal Society Publishing has made over 60k articles in their archive available for free.

SpringerOpen is the division of Springer dedicated to open access science, technology, and medicine.

ZooKeys is a peer-reviewed open access academic journal of zoology.

As a last resort, you can always post a request on the BioCurious mailing list - many of us do have institutional access to journals.