National Council of Teachers of Mathematics - This website has a wealth of mathematics lesson plans, research and information, but there is a subscription price.
Dositey com - Free educational programs, games, and printable worksheets in Math and English for grades K-8.
Cool math - An amusement park of math and more! Coolmath has a ton of cool math games and puzzles, lots of really clear math lessons (Algebra and fractions), fractals, Calculus, Geometry, and more.
A+ Math - Helps students improve their math skills interactively with flashcards and games. Also provides worksheets and homework help.
AAA Math - Math topics with interactive practice, explanations and challenge games for each.
Gamequarium: A portal to hundreds of free, online learning games. Game links for K-6 students arranged in more than 30 categories.
Kids Bank - Use your mouse and move it over any of our KidsBank.com™ characters! ... C'mon in and learn all about banking at KidsBank.com™!
http://www.kidsbank.com/index_3.asp
Math.com - World of Math Online - Offers free math lessons and homework help, with an emphasis on geometry, algebra, statistics, and calculus. Also provides calculators and games.
Mrs. Glosser's Math Goodies is a free math help site featuring interactive lessons, puzzles and worksheets. We have provided free homework help since 1999! There are more than 400 pages of activities for students, educators and parents.
National Library of Virtual Manipulatives is a digital library containing Java applets and activities for K-12 mathematics.
http://nlvm.usu.edu/en/nav/vlibrary.html
Illuminations - Illuminations is a partnership between the National Council of Teachers of Mathematics and MarcoPolo. There are many lessons and resources for teachers.
http://illuminations.nctm.org/
Shodor -To advance science and math education through the use of computational science, modeling and technology.
Math dictionary for kids - Every explanation kids want to know in math.
http://www.amathsdictionaryforkids.com/
Game Goo - Educational games to help develop early reading skills.