An affiliate of Stanford University, HighWire works to "facilitate the digital dissemination of more than 3000 journals, books, reference works, and proceedings." The advanced search feature allows you to specify exactly what you are looking for and check through all resources or only open access resources.
Pubget allows you to browse by paper or journal, offering you the option of viewing the PDFs of free-access articles or buying the PDFs of articles behind a paywall. According to the site, 10,000 new papers are added each day.
This database of journals published by Oxford University Press acts as an archive of over 300 journals in a wide range of subjects. You can search by subject or keyword to find your desired subject, and full texts are provided when available.
The mission of the Open Library of Humanities is to "support and extend open access to scholarship in the humanities." To research your topic, browse the featured journals and search for your required keywords.
PLOS is a non-profit organization committed to facilitating openness in academic research. According to the site, "all PLOS content is at the highest possible level of open access, meaning that scientific articles are immediately and freely available to anyone, anywhere."
Archives Hub allows you to search subjects, finding relevant information stored in various archives around Scotland, England, and Wales. Be aware that many of these are physical materials only available at certain institutions, but the site does provide some links to digital content, including images linking to full-resolution display and links to full-text files and PDF files, when available.
PubMed® comprises more than 33 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full text content from PubMed Central and publisher web sites.
Bioline International is a resource tool that provides access to open access research from developing countries. This is a great resource to find authoritative sources on health, biodiversity, environmental changes, conservation, and international development.
Although primarily a job-search tool, this site also allows you to search for recent articles by subject. Not only can you do your research here, but you can also sign up and customize a newsfeed that will deliver you articles of interest.
The International Aging Research Portfolio (IARP) is a system that organizes and allows users to browse research related to aging. It "incorporates databases of grants from multiple sources," as well as MEDLINE publication abstracts.
OAlster is a database that "contains records of digital resources from open-archive collections worldwide." You can become a member to get access to all the resources, and physical copies can be delivered to your library through WorldCat.
The National Criminal Justice Reference Service provides articles and other resources focused on criminal justice and drug-related information. It is funded by the U.S. government and provides links to full-text articles whenever possible.