The Mathematical Atlas (http://www.math-atlas.org/welcome.html)
Explanatory reference on the field of mathematics.

Mathworld (http://mathworld.wolfram.com/)
Encyclopedia of mathematical terms and concepts.

American Mathematical Society (http://www.ams.org/journals)
Full-text access to eight journals published by the American Mathematical Society.

arXiv.org (http://arxiv.org)
Articles on computative biology, physics, math, and computer science.

Cornell University Library Historical Math Monographs (http://historical.library.cornell.edu/math/)
Digitized math texts from the Mathematics Library of Cornell University.

Electronic Library of Mathematics (http://www.emis.ams.org/ELibEMS.html)
Full-text articles listed by publication.

GrayLIT Network (http://www.osti.gov/graylit/)
Government technical reports.

NUMDAM (http://www.numdam.org/en/)
Full-text articles from French mathematics journals.

University of Michigan Historical Math Collection(http://www.hti.umich.edu/u/umhistmath/)
Digitized math texts from the University of Michigan Library. To read the articles, add them to your “bookbag,” then download.