Interactive Math Manipulatives 

Toy Theater - (Elementary) Spinner, Fraction Bars, Counters, Base Ten Blocks, Ten Frames, and more.

National Library of Virtual Manipulatives - (K-12) Seperated by grade levels and domains. Everything you need from building shapes, Pentominoes, and Clocks, to Function Machines, Algebra Balance Scales, and Pythagorean Theorem.

Math Playground - (K-8) Geoboard, Pattern Blocks, Transformations, Fractions Tangrams, Factor trees, Functions, and more.

Glencoe - Secondary manipulatives.  Algebra, geometry.