Best Choices:
MathSciNet
Articles, reviews, abstracts, and citations dealing with mathematical sciences, provided by the American Mathematical Society.
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The Directory of Open Access Journals. Covers free, quality-controlled scientific and scholarly journals. Includes popular science magazines and professional journals. Scientific and technical content published by the IEEE (Institute of Electrical and Electronics Engineers) and its publishing partners.
DOAJ
General Science Full Text
IEEE Xplore