General Math Sites
AAA Math - divided by grade level or topic. Includes explanations, practice, games.
Aplusmath - flashcards, games, homework helper, worksheets.
Count On (games, activities, events)
Math Baseball - math problems in the form of a baseball game for 1 or 2 players
Mathline - lesson plans with video grades 6-8 (PBS)
Math, Science, and Technology Education (interdisciplinary projects)
Purple Math (algebra)
Wolfram Alpha - computational knowledge engine
Measurement
All About Measurements - includes metric converter
Fractions
Thirteen Ways of Looking at a Half
Estimation
Glowla's Estimation Contraption
Decimals
Decimals (practice)
Geometry
Games
Decimal and Whole Number Jeopardy (requires flash)
Graphing
Sports and Math
Baseball Simulations (decimals, angles, ratio)
Will Women Athletes Ever Catch and Surpass Their Male Counterparts?
Other Sports sites to use for statistical activities
National Baseball Hall of Fame
Guinness World Records: Sports and Games
For Teachers
Math Ideas (K-6)