Interactive online

Games

SET

4 × 4 × 4 Tic Tac Toe

and another version

Nim

and another Nim

Mastermind

Make 24

Puzzles

Sudoku

Tower of Hanoi

and another

Find the fake coin in 3 weighings

Ivars Peterson's MathTrek - The Counterfeit Coin

see also

Tools

Intelligent Online Graphing Calculators: WebGraphing.com

Illuminations: Paper Pool

Illuminations: Turtle Pond

Illuminations: Shape Tool

Illuminations: Isometric Drawing Tool

Illuminations: Geometric Solids

Illuminations: Circle Grapher

Illuminations: Bar Grapher

Manipulatives

National Library of Virtual Manipulatives

Geoboard from Interactive Mathematics Miscellany and Puzzles

Flexagon Portal - Home

Murderous Maths: The Fabulous Flexagons

Video: How to Flex a Hexaflexagon