American Mathematical Society - http://www.ams.org/
This the American Mathematical Society's gateway for delivering electronic products and services to mathematicians and engineers including books and journals in print; electronic journals on the e-Math home page; the MathSci database in several electronic formats
Ask Dr. Math - mathforum.org/dr.math
Ask Dr. Math is a question and answer service for math students and their teachers. A searchable archive is available by level and topic, as well as summaries of Frequently Asked Questions.
Furman University Mathematical Quotations Server - http://math.furman.edu/~mwoodard/mquot.html
A collection of mathematical quotations culled from many sources searchable by keyword or accessible in alphabetic order by the author's last name.
GAMS - Guide to Available Math Software - http://gams.nist.gov/
This is developed by the National Institute of Standards and Technology. It is a cross-index and virtual repository of mathematical and statistical software components of use in computational science and engineering.
History of Mathematics Archive - http://www-groups.dcs.st-and.ac.uk/~history/
This site contains biographies of over 1,100 mathematicians from around the world. Sponsored by the University of St. Andrews in Scotland.
Mathtools.net - http://www.mathtools.net/
A free technical computing portal for scientific and engineering needs with links to technical computing programmers, covering MATLAB, Java, Excel, C/C+, Fortran and others.
Society for Industrial and Applied Mathematics (SIAM) - http://www.siam.org/
The organization of applied mathematicians and computational scientists, dedicated to enriching the profession through publications, conferences, activity groups, and programs since 1952.