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
The organization of applied mathematicians and computational scientists, dedicated to enriching the profession through publications, conferences, activity groups, and programs since 1952.
Wolfram|Alpha is a computational knowledge engine: it generates output by doing computations from its own internal knowledge base, instead of searching the web and returning links. Give it a try, it's fantastic!
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.
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.