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.
1868-1930. Excellent bibliography for older mathematics research, the European Mathematical Society's online version of Jahrbuch uber die Fortschritte der Mathematik covers also has links to scanned images of classic papers. Older journals are also indexed prior to 1860's, e.g. Crelle's journal (Journal für die Reine und Angewandte Mathematik) has been extended to Vol. 1 (1826).
Scopus is an abstract and citation database of peer-reviewed research literature with more than 20,500 titles from more than 5,000 international publishers. Scopus offers researchers a quick, easy and comprehensive resource to support their research needs in the scientific, technical, medical and social sciences fields and arts and humanities.
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.