Google Scholar: https://scholar.google.com/ - Broad search for scholarly literature, articles, theses, books.
JSTOR: https://about.jstor.org/librarians/books/open-access-books-jstor/ - Access to peer-reviewed journals, books, and primary sources (often via campus login).
PubMed Central (PMC): https://pmc.ncbi.nlm.nih.gov/ - Free full-text archive of biomedical and life sciences journal literature.