SHERPA/RoMEO Check the self-archiving and open access policies of publishers.
SHERPA/Juliet Check the open access policies of funders.
SHERPA Fact is a tool to help researchers check if the journals in which they wish to publish their results comply with selected funder's requirements.
Directory of Open Access Journals (DOAJ) Search or browse open access journals.
Directory of Open Access Books Search or browse books that have been published in open access.
OpenDOAR Directory of open access repositories.
Registry of Research Data Repositories Search or browse data repositories by subject, content type or country. Not all repositories are open access.
The Can I Share It look-up tool provides an easy way to check where a journal article can be shared in line with the paper’s access and usage rights.
AgEcon
Electronically distributes reports of scholarly research in the field of agricultural economics.
arXiv
Open access to e-prints in Physics, Mathematics, Computer Science, Quantitative Biology and Statistics.
ChemRxiv
ChemRxiv is a free online submission, distribution, and archival service for unpublished preprints in chemistry and related areas.
Cogprints
An electronic collection of self-archived papers in areas related to cognition including: psychology, neuroscience, linguistics, computer science, philosophy, and biology
Disciplinary Repositories
The Open Access Directory (OAD) is a compendium of simple factual lists about open access (OA) to science and scholarship, maintained by the OA community at large.
EarthArXiv
A non-profit preprint server for the Earth Sciences. Part of the Center for Open Science network.
LawArXiv
LawArXiv is a free, open access, nonprofit pre & post print repository for scholarly legal content. It is owned and maintained by members of the legal community.
Nature Proceedings
A free service started in January 2007. It is a permanent, citable archive for pre-publication research and preliminary findings in biology, medicine (except clinical trials), chemistry and earth sciences.
Open Science Framework
The Center for Open Science is a non-profit technology organization whose mission is to "increase the openness, integrity, and reproducibility of scientific research."
PsyArXiv
A free preprint service for the psychological sciences.
SocarXiv
SocArXiv is an online preprint server for the social science
Browser extensions can search for Open Access versions of journal articles when you encounter paywalls.