Reference

Digital Library of Mathematical Functions
This mockup gives an idea of the current ideas about the design and organization of the new Digital Library; these ideas are, of course, subject to change. Content only is only available in a few places.
http://dlmf.nist.gov/

EquationSheet.com
Searchable directory, arranged by field, of equations, constants, symbols and SI units. Also features LaTEX utilities, software links.
http://www.equationsheet.com/

Geometry Formulas and Facts
An excerpt from the 30th Edition of the CRC Standard Mathematical Tables and Formulas, covering the area of Geometry (minus differential geometry), by Silvio Levy.
http://geom.math.uiuc.edu/docs/reference/CRC-formulas/

Glossary of Math Terms
Explains commonly used words specialized to this field. Includes examples and some graph illustrations.
http://www.dpi.state.wi.us/dpi/standards/mathglos.html

Math Assistance
Lists rules and formulas for a number of mathematical subjects, such as plotting graphics, functions, factoring, derivatives, integrals, matrices, vectors, and numerical analysis. In English, French and Turkish languages.
http://www.ercangurvit.com/

The Math Reference Project
An electronic archive of articles by Karl Dahlke on topics ranging from high school geometry up to graduate level topology.
http://www.mathreference.com/

Mathematical Atlas
A topical guide to modern mathematics that explains the subdisciplines. Includes graphics that represent their relative size and relationship.
http://www.math.niu.edu/~rusin/known-math/welcome.html

Mathematics Resources
This site comprises a large number of links to matheamtical resources, dedicated to various topics.
http://www.educationindex.com/math/

Math2.Org
Formerly Dave's Math Tables. Mathematical tables on-line and downloadable.
http://www.math2.org/

On-Line Encyclopedia of Integer Sequences
Identify a sequence by entering few terms in the sequence. Additional information includes examples, formulas, and related links.
http://www.research.att.com/~njas/sequences/

PlanetMath
A collaborative encyclopaedia with entries contributed under the GNU Free Documentation License.
http://planetmath.org/

Platonic Realms
Online resource for the mathematics community: links library, bookstore, quotes, and online articles.
http://www.mathacademy.com/pr/index.asp

Summation Formulae
For polynomials of degrees up to 50, generated using the Euler-Maclaurin method.
http://polysum.tripod.com/

Wikipedia: Mathematics Section
Free encyclopedia. Cover number systems, different math fields, foundations, history, applications, and famous problems.
http://en.wikipedia.org/wiki/Mathematics/

Wolfram: Mathematical Functions
Includes advanced formulas and visualizations of various equation, such as elliptical curves.
http://functions.wolfram.com/