MathSciNet is a comprehensive database covering the world's mathematical literature since 1940, providing Web access to the bibliographic data and reviews of mathematical research literature contained in the Mathematical Reviews Database.
Pre-print server for Physics, Mathematics, Nonlinear Sciences, Computer Science, Quantitative Biology Open access repository that includes thousands of physics and math articles.
This multidisciplinary database covers the major journals in mathematics. You can also see an article's reference list, as well as its citing references (who has cited that article).
Web of Science
This is another good multidisciplinary database for mathematics. It also lets you see an article's reference list and citing references.