Mathematics has it's own Subject Classification system to help people find articles and other materials on specific subjects. Mostly used in Mathematical Reviews (MathSciNet) and Zentrallblatt MATH, it is a useful classification system to know. An introduction can be found at A Gentle Introduction to the Mathematics Subject Classification Scheme.
- Mathematical Reviews
The best database for English language reviews and commentary on the Mathematical literature. Also called MathSciNet, it includes Current Mathematical Publications.
- EuDML -- the European Digital Mathematics Library
~250 archival European mathematics journals
- ISI Web of Knowledge
The best general science database.
- Mathematics at ArXiv.org
The universally used preprint archive, this is the best place to get brand new papers, before they are published.
- The Jahrbuch Project
It consists of a complete electronic catalogue of the mathematical publications in the time period between 1868 - 1942 (JFM database) and a digital archive storing the most relevant publications from that period. The two components are linked very closely
- The On-Line Encyclopedia of Integer Sequences
Type in the beginning of a sequence, get all the information about it.
- Zentralblatt MATH
Zentralblatt MATH is the world's most complete and longest running abstracting and reviewing service in pure and applied mathematics.
JSTOR is an online storage facility for older journals in many disciplines. This link brings you to a list of all the Mathematics titles and years that are in the JSTOR collection.