Clay Mathematics Institute http://www.claymath.org/index.php

The Clay Mathematics Institute is a privately funded operating foundation dedicated to increasing and disseminating mathematics. The Institute operates in programs that introduce high school students to mathematical research: the Clay Research Academy, the Ross and PROMYS programs. CMI organizes public lectures and publishes conference proceedings and monographs.

MathWords.com http://home.austin.rr.com/bsimmons/mathwords/mathwords_home.html

This site contains selections from the full-length reference book MathWords - Terms and Formulas from Algebra I to Calculus

Euclid's Elements http://aleph0.clarku.edu/~djoyce/java/elements/elements.html

Euclid's Elements form one of the most beautiful and influential works of science in the history of humankind. Its beauty lies in its logical development of geometry and other branches of mathematics. It has influenced all branches of science but none so much as mathematics and the exact sciences. The Elements have been studied 24 centuries in many languages starting, of course, in the original Greek, then in Arabic, Latin, and many modern languages.

Mathematical Quotations Server http://math.furman.edu/~mwoodard/mquot.html

Furman University hosted site that has a searchable database of quotations that refer to mathematics.

Mathway http://www.mathway.com Mathway solves your math problems with step-by-step explanations. It is for students, parents, and teachers, and covers the following math subjects: Basic Math | Pre-Algebra | Algebra | Linear Algebra | Trigonometry | Precalculus | Calculus. Mathway also provides Graphing and Glossary definitions.

Interactive Mathematics http://www.intmath.com

Learn maths while you play with it! Uses LiveMath plugin, Scientific Notebook and Flash.

MathNerds http://www.mathnerds.com

MathNerds provides Free, discovery-based, mathematical guidance via an international, volunteer network of mathematicians.

The Math Forum http://mathforum.org

The Math Forum is a leading center for mathematics and mathematics education on the Internet. The Math Forum's mission is to provide resources, materials, activities, person-to-person interactions, and educational products and services that enrich and support teaching and learning in an increasingly technological world.

Webmath (Discovery.com) http://www.webmath.com

Unlike most math sites, Webmath is not a database of questions and answers nor is it an online testing site. At Webmath, you don't have to wait for an e-mail response or post your question to an electronic bulletin board to get math homework help. You can get the answer to your math problem right here, right now.

MATHguide http://www.mathguide.com

A mathematics resource center globally available to students, parents and teachers.

Mathematrix http://math.dartmouth.edu/~leejstem/mathematrix/index.html

A web site devoted to exploring mathematical recreations - the more entertaning (and generally lesser known) areas of mathematics.

MathPuzzle.com http://www.mathpuzzle.com/

Martin Gardner celebrates math puzzles and Mathematical Recreations. This site aims to do the same.

Problem of the Week http://www.wits.ac.za/ssproule/pow.htm

San Jose Math Circle/Bay Area Mathematical Adventures http://www.geometer.org/sjcircle/

Mathematical circles are weekly gatherings of students of high school age or younger working on problems involving complex and advanced topics, guided by mathematicians and educators. 

Missouri State University's Problem Center http://math.smsu.edu/~les/POTW.html

There are three levels of problems: High School, Advanced, and Challenge. There are also archives of past problems. Problems are updated monthly & solutions will appear approximately two months after the problems have been posted.

Nrich http://www.nrich.maths.org.uk/public/index.php

Part of The Millennium Mathematics Project (MMP) which is a maths education initiative for ages 5 to 19 and the general public, based at the University of Cambridge but active nationally and internationally.

Nick's Mathematical Puzzles http://www.qbyte.org/puzzles/

American Regions Math League http://www.arml.com/

ARML is the World Series of mathematics competitions. The contest is written for high school students, although some exceptional junior high students attend each year. The competition consists of several events, which include a team round, a power question (in which a team solves proof-oriented questions), an individual round, two relay rounds (in which a contestant solves a problem and passes his/her answer to another team member, who uses this answer to solve another problem), and a super relay.

NCTM High School Challenge Problems http://www.nctm.org/high/challenge/index.htm

Mu Alpha Theta http://www.mualphatheta.org/index.htm

Mu Alpha Theta is a National Mathematics Honor Society encompassing over 65,000 students in more than 1,400 schools. We are dedicated to inspiring keen interest in mathematics, developing strong scholarship in the subject, and promoting the enjoyment of mathematics in high school and two-year college students.

The X + Y Files http://www.counton.org/xplusyfiles/

The Four Color Theorem http://www.math.gatech.edu/~thomas/FC/fourcolor.html

This page gives a brief summary of a new proof of the Four Color Theorem and a four-coloring algorithm found by Neil Robertson, Daniel P. Sanders, Paul Seymour and Robin Thomas.

Number Theory

Prime Numbers http://www.svobodat.com/primes/

Lists several thousands of prime numbers . . . and advertisements for cryptography books.

The Great Internet Mersenne Prime Search (GIMPS) http://www.mersenne.org/

Modular Arithmetic Pages http://www.math.csusb.edu/faculty/susan/modular/modular.html

On these pages you can learn about modular arithmetic, which is arithmetic on a circle instead of a number line.

Historical Topics in Number Theory http://www-gap.dcs.st-and.ac.uk/~history/Indexes/Number_Theory.html

From the MacTutor History of Mathematics archive

The British Society for the History of Mathematics http://www.dcs.warwick.ac.uk/bshm/index.html

The aims of the British Society for the History of Mathematics are to promote research into the history of mathematics and its use at all levels of mathematics education. It does this through meetings, a newsletter and by other means. The BSHM takes in all periods and civilizations, and all aspects of mathematics, both pure and applied. Members come from a broad cross-section of those interested in mathematics, its history and education, and from many countries.

Mathematics Archives http://archives.math.utk.edu/topics/history.html

A collection of links on the history of mathematics.

The Math Forum http://mathforum.org/library/topics/history

A collection of links on the history of mathematics.

The History of Mathematics http://www.maths.tcd.ie/pub/HistMath/

A collection of links on the history of mathematics maintained by David R. Wilkins of Trinity College, Dublin

The MacTutor History of Mathematics Archive http://www-groups.dcs.st-andrews.ac.uk/~history/

A collection of links on the history of mathematics maintained by the School of Mathematics and Statistics at the University of St. Andrews in Scotland.

Graphmatica http://www8.pair.com/ksoft/

A powerful, easy-to-use, equation plotter with numerical and calculus features. Graph Cartesian functions, relations, and inequalities, plus polar, parametric, and ordinary differential equations.

Educational Java Programs http://ejad.best.vwh.net/java/

This site was started in November of 1997 with the purpose of providing Java™ Applets that can be used as tools to help and enhance the education of children both at school and at home.

Cut The Knot! http://www.cut-the-knot.com/ctk/index.html

An interactive column using Java applets

The Probability/Statistics Object Library http://www.math.uah.edu/psol/

The Probability/Statistics Object Library is a resource for teachers and students of probability and statistics.

Workshop Statistics http://www.keycollege.com/workshopstatistics/vonoehse/index.html

Written by Allan J. Rossman and J. Barr von Oehsen ; Published by Springer-Verlag New York, Inc., 1997 ; Distributed by Key Curriculum Press

The Probability/Statistics Object Library http://www.math.uah.edu/psol/

The Probability/Statistics Object Library is a resource for teachers and students of probability and statistics.

Berinstein Research: Directory of Online Statistics Resources http://www.berinsteinresearch.com/stats.htm

This directory lists online statistics sources, by subject, for quick reference.

Sample Size Calculator http://www.surveysystem.com/sscalc.htm

This Sample Size Calculator is presented as a public service of Creative Research Systems.  You can use it to determine how many people you need to interview in order to get results that reflect the target population as precisely as needed.  You can also find the level of precision you have in an existing sample.

American Statistical Association http://amstat.org/

To be a world leader in promoting statistical practice, applications, and research; publishing statistical journals; improving statistical education; and advancing the statistics profession.

Mudd Math Fun Facts http://www.math.hmc.edu/funfacts/

Ideas and puzzles that will change the way you think. Designed as a resource for enriching your math courses and nurturing your interest and talent in mathematics! Each Math Fun Fact is a math puzzle or short article that contains a cool mathematics idea. You'll can learn about the mathematics of things like card shuffling to poker to computer vision to fractals to music, just to name a few.

Math Songs http://www.tomdukich.com/math%20songs.html

Audio and video pieces based on five famous mathematical constants. The constants were "sonified" by assigning notes from various musical instruments to the digits 0 through 9.

Annenberg Media http://www.learner.org/index.html

Teacher resources and teacher professional development programming across the curriculum

Learning Math (Annenberg Media) http://www.learner.org/channel/courses/learningmath/index.html

Features 5 college-level math courses for elementary and middle school teachers.