Directory of Open Access Journals (DOAJ) -- Covers free, full-text, quality-controlled scientific and scholarly journals.
OSF Preprints -- Preprint repository from the Open Science Framework. Covers a variety of disciplines, not just science.
Paperity -- Multidisciplinary open access article search engine. Publishers and journals pay a small fee to be indexed.
Social Science Research Network --The SSRN is an organization dedicated to the broad dissemination of social science research material. They maintain a database of abstracts of current and forthcoming research papers & Electronic Paper Collection of full-text articles by current scholars.