The arXiv is an archive for electronic preprints of scientific papers in the fields of mathematics, physics, astronomy, computer science, quantitative biology, statistics, and quantitative finance which can be accessed online.
MathSciNet is a comprehensive database covering the world's mathematical literature since 1940 and provides access to data from Mathematical Reviews (updated monthly) and Current Mathematical Publications (updated daily). To search this database by topic, click "Free Tools." This will let you enter your keywords, and then it will map them to the MSC code actually used by the database.
ScienceDirect is a web information source for scientific, technical, and medical research (STM) journals across 24 fields of science including the social sciences. ScienceDirect offers full-text articles, browsable tables of contents,and article abstracts.
Compendex is the most comprehensive bibliographic database of scientific and technical engineering research available, covering all engineering disciplines. It includes millions of bibliographic citations and abstracts from thousands of engineering journals and conference proceedings.
Use the limiters on the left hand side of the search tab to limit first to journals, then to the subject Mathematics in order to retrieve a list of the 604 mathematics journals offered via open access.