Colorful Mathematics
Educational software presents advanced mathematics to K-12 students in a game-oriented approach, using coloring and drawing techniques to illustrate concepts from graph theory and computer science.
http://www.math.ucalgary.ca/~laf/colorful/colorful.html

Pythabacus
Description and instruction of using the ancient Pythagorean Abacus to do arithmetic.
http://www.pythabacus.com/

Mathwizz
Interactive math problems for abilities ranging from middle school to university.
http://www.mathwizz.com/

MatheMagic
A collection of mathematical puzzles, jokes and articles.
http://geocities.com/mathimoh/

Mathschallenge.net
A website dedicated to the puzzling world of mathematics, with a discussion forum.
http://mathschallenge.net/

Brain Teasers and Maths Puzzles
Interactive logical and mathematical puzzles with feedback and detailed solutions.
http://www.syvum.com/teasers/

Logicville
Educational puzzles, brain teasers, and word games such as anagrams, cryptograms, alphametics, doublets, tangrams.
http://www.logicville.com/

MatheMajik
Indian site featuring calculation tricks and math puzzles.
http://mathemajik.tripod.com/

Polymorf - Symmetry of Polyhedra
A demonstration of the symmetry and transformational properties of the Platonic and Archimedean solids.
http://www.polymorf.net/

Mr Titen's More Than Math
Mathematics teacher provides puzzles for students.
http://www.geocities.com/EnchantedForest/Tower/1133/

The Puzzling World of Barry R. Clarke
Brain teasers, articles and puzzles forum.
http://barryispuzzled.com/

Brain Teasers by QBrute
Original brain teasers with solutions.
http://www.angelfire.com/empire/qbrute/

Magic Math Kingdom
Learn math through games and quiz-oriented tutorial.
http://www.nes-lab.com/english2/

The Enigmatic World of Philip Carter
Collection of brain teasers from the author of IQ books.
http://www.knowl.demon.co.uk/

Perfect Times
Teaches and assesses fluency of multiplication and division tables using online cards. Submit data and compare results.
http://www.perfect-times.co.uk/

Mixed Up Math
Logic problems, where equations are mixed up. In various formats like boxes, crosses, diagonals, mazes. Free download.
http://www.kprobe.com/kprobe/mu.htm

Wu's Riddles
A collection of riddles, puzzles, and technical interview questions, with a discussion forum.
http://www.ocf.berkeley.edu/~wwu/riddles/intro.shtml

Mathematical Recreations
Detailed exploration of a selection of mathematical topics.
http://www.mathrec.org/

Nick's Mathematical Puzzles
A collection of puzzles, with hints, solutions, and links to related mathematical topics.
http://www.qbyte.org/puzzles/

Paul Hsieh's Puzzles
Logical and mathematical brain teasers, with an indication of difficulty. Hints are provided; solutions are available by email.
http://www.azillionmonkeys.com/qed/puzzles.html

The Contest Center
Classic puzzles and riddles requiring only simple logic and elementary math.
http://www.contestcen.com/classic.htm

15 Puzzle of Sam Lloyd
Play online Sam Lloyd's "15" puzzle. The funny story and mathematical implications of this game. Some funny logical problems with the 15 game.
http://www.holotronix.com/samlloyd15.php

Torus and Klein Bottle Games
Collection of Java-based educational puzzles.
http://www.geometrygames.org/TorusGames/index.html

Math Puzzles
Logic puzzles and riddles with some solutions.
http://www.math.utah.edu/~cherk/puzzles.html

Alien Tiles
Object of puzzle is to try to get color pieces to create certain patterns in cube.
http://www.alientiles.com/

Alphametic Puzzles
Numerous puzzles, links, puzzle solver, and a generator.
http://www.tkcs-collins.com/truman/alphamet/

Scott Kim
Puzzles for web, computers, and print; ambigrams.
http://www.scottkim.com/

Contest Center Cryptarithms
Examples of cryptarithms, with prize competitions for subscribers.
http://www.contestcen.com/rithms.htm

Alphametics Page
Compiled by Mike Keith. Examples of various genres.
http://members.aol.com/s6sj7gt/mikealp.htm

Logic Puzzles
Collection of original numerical-logic puzzles. Many are long and difficult, involve sorting and comparing long lists and are intended to be solved using a good spreadsheet program.
http://www.geocities.com/bob_kraus_2000/

A Primer on Cryptarithmetic
Instructions, examples, references and links.
http://www.geocities.com/Athens/Agora/2160/primer.html

Mathpuzzle.com
Celebrates math puzzles and mathematical recreations.
http://www.mathpuzzle.com/

Algebraic Math Puzzles
A collection of puzzles at high school level. Does not include answers.
http://www.lifesmith.com/mathpuz1.html

Free Puzzle Collections
Educational and recreational puzzles for teachers, students and parents. Each puzzle is graded by difficulty and has a hint. Solutions are available to members.
http://www.freepuzzles.com/

Aha! Puzzles
Puzzles, books, games and tests with designed by Lloyd King, author of "Test Your Creative Thinking".
http://www.ahapuzzles.com

Storkey: Two Envelope Paradox
A tough but simply defined problem which demonstrates key issues in probability.
http://www.anc.ed.ac.uk/~amos/doubleswap.html

Mathproblems.info
Over one hundred and fifty math problems ranging from basic math to differential equations.
http://mathproblems.info/

Nick's Mathematical Puzzles
A collection of puzzles, with hints, solutions, and links to related mathematical topics.
http://www.qbyte.org/puzzles/rss2.xml

Puzzle of the Week Archives
A collection of puzzles, with hints and full solutions. Compiled by George Moody.
http://ecg.mit.edu/george/puzzles/

Polyhedra Links
Pastime and activity links for various geometric patterns, polygons and polyhedra by Jill Britton.
http://britton.disted.camosun.bc.ca/jbpolyhedra.htm

Ben Robinson's Cross-Number Puzzles
A collection of cross-number puzzles: similar to crosswords, but using digits instead of letters. Many involve complex mathematical reasoning.
http://www.users.bigpond.com/bendr/xwd/numeric/index.html

Puzzles of Leonid Mochalov
A variety of challenging geometrical and arithmetical puzzles, many with solutions.
http://puzzlemochalov.com/

Mathschallenge.net
A website dedicated to the puzzling world of mathematics.
http://mathschallenge.net/rss2.xml

Nandor's Exhaustive Chemical Words Pages
Lists all the English words, word squares, and magic squares that can be written using chemical symbols.
http://www.wellington.org/nandor/chemwords/chemwords.htm

Group Games
Animations that develop the theory behind such puzzles as the Rubik cube.
http://www.dmi.units.it/~logar/duek2/index_engl.html

Downloadable and online games
A variety of Flash- and Java-based games and puzzles.
http://www.gameintellect.com/

Interactive Mathematics Miscellany and Puzzles
Java-based math resource including articles, puzzles and links.
http://www.cut-the-knot.org/

AIMS Puzzle Corner
Provides over 100 puzzles with the goal of creating an engaging learning environment.
http://www.aimsedu.org/puzzle/

Calculus-Help Problem of the Week Archive
Calculus based problems range from easy to difficult.
http://www.calculus-help.com/problem.html

The Little Math Puzzle
The Little Math Puzzle has been a K-12 Internet activity since 1995 for grades 5-9. Currently the puzzles have ceased but the site maintains complete archives of past problems.
http://www.sprucelog.com/lmp/

Mathematical Games
A collection of pencil and paper games with mathematical content. Many can be played through the site.
http://www.madras.fife.sch.uk/maths/games/

Dick's Math Puzzle Page
A collection of mathematical puzzles, some original, with fully worked solutions.
http://www.mod2.org/

Creative puzzles
Interactive puzzle archive with descriptions for puzzle construction and a special children's puzzle page.
http://www.creatievepuzzels.com/