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3:4:5 Triangle - Interactive demonstration of the Pythagorean Theorum
National Library of Virtual Manipulatives
NCTM Illuminations By: Marcopolo - 96 Online activities related to math
Animated Math GIFs - from Wolfram MathWorld
CoolMath - Online games lessons and calculators
Problems with a Point - Find problems you can use in your classroom.
Nick's Mathematical Puzzles - A collection of puzzles, hints, solutions, and links to related topics
NCTM Electronic Examples - Activities that Focus on the National Standards
Interactive Mathematics - Learn math by playing with it.
MathWorld