Interesting Sites
Loki 3 Flexagons
Dozens of different flexagons, with downloads and folding instructions for almost all of them. Most flexagons even have quite a bit of documentation about them, including observations and notation.
The Flexagon Portal
Guides for a few flexagons, including the dodecahexaflexagon (a hexaflexagon with 12 sides).
The Hexahexaflexagon
A quick and easy guide to making a hexahexaflexagon (a hexaflexagon with 6 sides).
Wolfram MathWorld Elementary Cellular Automaton
An enciclopedia for the elementary cellular automaton that fully lays out all its rules and results.
Elementary Cellular Automatan Online
A bare bones site to simulate the elementary cellular automaton.
3D Cellular Automatan
A 3D version of the elementary cellular automaton, with many settings to choose from.
Big Number Calculator
A calculator capable of working with very large numbers.
Chrome Music Lab
Some simple music and sound makers to experiment with.
Science vs Magic
A few extremely well made geometry demonstrations.
Cryptii Encryptor
Modular text encryption and decryption online