BASE (Bielefeld Academic Search Engine)
BASE is one of the world's most voluminous search engines especially for academic web resources. BASE provides more than 100 million documents from more than 5,000 sources. You can access the full texts of about 60% of the indexed documents for free (Open Access). BASE is operated by Bielefeld University Library.
DOAJ
DOAJ is an online directory that indexes and provides access to high quality, open access, peer-reviewed journals.
All journals that were accepted into DOAJ before March 2014 are now required to reapply.
DOAB (Directory of OA Books)
DOAB has 5297 Academic peer-reviewed books from 160 publishers that are in the public domain. Enter and search by key word. Or use this spreadsheet of the titles sorted by LC classification.
arXiv
arXiv is a free archive of articles and preprints in Physics, Mathematics, Computer Science, and Quantitative Biology.
PubMed
PubMed Central is the free archive of mostly biomedical and life sciences journal literature from the U.S. National Institutes of Health (NIH).