AAA Math - divided by grade level or topic.  Includes explanations, practice, games.
http://www.aaamath.com/

A+ Math - flashcards, games, homework helper, worksheets.
http://www.aplusmath.com/

Ask Dr. Math
http://mathforum.org/dr.math/

Cool Math for Kids
http://www.coolmath4kids.com/

Count On (games, activities, events)
http://www.mathsyear2000.co.uk/

Fact Monster: Math
http://www.factmonster.com/mathmoney.html

Math Baseball -Math problems in the form of a baseball game for 1 or 2 players.
http://www.funbrain.com/funbrain/math/

MathDrill
http://www.mathdrill.com/

Mathline
http://www.pbs.org/teachersource/mathline/lessonplans/search_6-8.shtm

Math Forum @ Drexel
http://mathforum.org/

Shodor Interactivate
http://www.shodor.org/interactivate/

Math, Science, and Technology Education (interdisciplinary projects)
http://www.mste.uiuc.edu/

Math Dictionary for Kids
http://www.amathsdictionaryforkids.com/

Purple Math
http://www.purplemath.com

Measurement

All About Measurements - includes a metric converter
http://www.aaamath.com/mea.html

Metric Units and Measurement
http://www.mathleague.com/help/metric/metric.htm

Fractions

Fraction Bars
http://arcytech.org/java/fractions/

Virtual Manipulative: Parts of a Whole - interactive tool
http://nlvm.usu.edu/en/nav/frames_asid_102_g_1_t_1.html

Pattern Blocks: Exploring Fractions With Shape
http://arcytech.org/java/patterns/

Thirteen Ways of Looking at a Half
http://pbskids.org/cyberchase/games/fractions/index.html

Compare Fractions
http://www2.whidbey.net/ohmsmath/webwork/javascript/fracstrp.htm

Estimation

Glowla's Estimation Contraption
http://pbskids.org/cyberchase/games/ballparkestimation/index.html

Estimator Four
http://www.shodor.org/interactivate/activities/egame/index.html

Estimator
http://www.shodor.org/interactivate/activities/estim/index.html

Decimals

Decimals (practice)
http://www.aaamath.com/dec.html

Geometry

Geometry Basics
www.aaamath.com/geo.html

Games 

Count On (games, activities, events)
http://www.mathsyear2000.co.uk/

Decimal Squares Interactive Games
www.decimalsquares.com/dsGames

Decimal and Whole Number Jeopardy (requires falsh player)
http://www.quia.com/cb/8142.html

Railroad Repair
http://pbskids.org/cyberchase/games/decimals/decimals.html

Shodor Interactive
http://www.shodor.org/interactive

Cyberchase Games Central
http://pbskids.org.cyberchase/games.html

Ordering Decimals

Builder Ted
http://www.bbc.co.uk/education/mathsfile/shockwave/games/laddergame.html

Graphing

Create a Graph
http://nces.ed.gov/nceskids/Graphing/

What's the Point?
http://www.funbrain.com/co/

Sports and Math

Baseball Simulations (decimals, angles, ratios)
http://www.exploratorium.edu/baseball/scientificslugger.html

Baseball Simulator
http://faculty.tcc.fl.edu/scma/carrj/Java/baseball4.html

Math: Baseball
http://www.funbrain.com/math/

Probability and the NBA Draft 
(use the info below to compare historical probabilities and fairness of current system)
http://www.nba.com/history/lottery_probabilities.html

Will Women Athlete Ever Catch and Surpass Their Male Counterparts?
http://www.pbs.org/teachersource/mathline/concepts/sportsandmath/activity1.shtm

Other Sports Sites to Use for Statistical Activities

Olympic Games History
http://www.olympic.org/uk/games/index_uk.asp

Major League Baseball
http://mlb.mlb.com/NASApp/mlb/mlb/stats/index.jsp

National Baseball Hall of Fame
http://www.baseballhalloffame.org/

Sports Records
http://www.sportsrecords.co.uk/

Guinness World Records: Sports and Games
http://www.guinnessworldrecords.com/

For Teachers

Principles and Standards for School Mathematics
http://standards.nctm.org/document/chapter1/index.htm

Math Ideas (K-6)
http://www.ilovethatteachingidea.com/ideas/subj_math.htm

Math Problems with a Point (NCTM honored site emphasizes problem solving)
http://www2.edc.org/mathproblems/default.asp

Illuminations: Marco Polo
http://illuminations.nctm.org/

National Library of Virtual Manipulatives
http://nlvm.usu.edu/EN/NAV/VLIBRARY.HTML