MathSciNet
The essential research tool indexing and reviewing the literature of the mathematical sciences with coverage from the 19th century to the present.
Zentralblatt fur Mathematik / Mathematics Abstracts
An abstracting and indexing service for pure and applied mathematics.
Google Scholar
A great tool that also does citation indexing.
Wikipedia
The articles are generally well written and authoritative.
MacTutor History of Mathematics archive
A source of biographies of mathematicians and the history of mathematics.
Scopus
A citation index covering a wide range of scientific literature with coverage of mathematics.
The Web of Science
A citation index covering a wide range of scientific literature but with limited coverage of mathematics.
Mathworld
This is a comprehensive and interactive mathematics encyclopedia.
Jahrbuch-Project Electronic Research Archive for Mathematics
A resource for searching mathematics literature from from 1868 to 1943.
arXiv
Started in 1991. Now located at Cornell, arXiv.org is a fully automated electronic archive and distribution server for research papers.
ERIC
This index covers periodicals and other materials in the field of Education, including Education in Mathematics.
Wolfram|Alpha
Based on Mathematica, this search engine allows you to enter a query and returns an answer from structured data.