National Library of Virtual Manipulatives for Interactive Mathematics
http://nlvm.usu.edu/en/nav/vlibrary.html

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

NCTM: Illuminations
http://illuminations.nctm.org/swr/index.asp
http://illuminations.nctm.org/tools/index.aspx
http://illuminations.nctm.org/

A Maths Dictionary for Kids
http://www.teachers.ash.org.au/jeather/maths/dictionary.html

Interactive Mathematics Miscellany and Puzzles
http://www.cut-the-knot.org/index.shtml

Interactive Mathematics Activities
http://www.cut-the-knot.org/Curriculum/index.shtml

AAA Math
http://www.aaamath.com/

Interactive Math Lessons, Tutorials, and Games Online
http://www.homeschoolmath.net/math_resources_3.php

MathScape
http://www.shodor.org/interactivate/texts/mathscape/