Coverage: current MathWorld is a comprehensive and interactive mathematics encyclopedia intended for students, educators, math enthusiasts, and researchers. Published as the CRC Encyclopedia of Mathematics (3rd ed. 2009).
Online library of resources for science, technology, engineering, and mathematics education and research.
Coverage: current The NIST Handbook of Mathematical Functions, together with its Web counterpart, the NIST Digital Library of Mathematical Functions (DLMF), is the culmination of a project that was conceived in 1996 at the National Institute of Standards and Technology(NIST). The project had two equally important goals: to develop an authoritative replacement for the highly successful Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, published in 1964 by the National Bureau of Standards (M. Abramowitz and I. A. Stegun, editors); and to disseminate essentially the same information from a public Web site operated by NIST.
Math Forum - best general math site on the Web. Gold mine for undergraduate students, as well as quality online mathematics education for professors.