The arXiv: A free online database with publications relating to physics, computer sciences, mathematics, finance and biology.
EconBiz: A free online database with full-text access to a multitude of economics-related publications.
ERIC Insttitute of Education Sciences: Thousands of publications relevant to education, human and natural sciences, the arts, and many more, often with free access to full texts or links to host sites.
Google Scholar: A search engine with a specific focus on scholarly articles, often accessible as full-text versions.
JURN: A search engine with a specific focus on free scholarly articles, accessible as full text versions.