Some appear in posts, others do not.

Constantly under construction...

Euclidea: An unbelievably intuitive game to explore geometric constructions.

https://www.euclidea.xyz/

A super snazzy collection of interactive simulations.

http://explorabl.es/

http://ncase.me/trust/

A helpful collection of puzzles and curiosities.

http://www2.stetson.edu/~efriedma/puzzle/index.html

A collection of circle graphs of the world's flags.

http://shaheeilyas.com/flags/

Problem Solving Classrooms. A guiding piece for grade 8.

http://blogs.sd41.bc.ca/math/files/2011/02/Teaching-Through-Problems-Worth-Solving-Grade-8-1.0-to-print.pdf

Math Hombre's page for Interactive resources.

http://mathhombre.blogspot.ca/p/online.html

Make Desmos gifs (from Chris Lusto @Lustomatical)

http://www.gifsmos.com

SolveMe Mobiles: Balancing Game

http://solveme.edc.org/Mobiles.html

Puzzle Game: Directing Lasers working with Fractions

http://centerforgamescience.org/blog/portfolio/refraction/

Current World Stats

http://www.worldometers.info

Half Full

http://apps.evanweinberg.org/halffull/

David Wees' Graph Game

http://davidwees.com/graphgame/

Don Steward's Resource Blog

http://donsteward.blogspot.co.uk

Multiplying Binomials w/ Algebra Tiles on Desmos

https://www.desmos.com/calculator/tfpuxpz9hr

Collection of Desmos polygraphs

https://teacher.desmos.com/search?q=polygraph

John Mason's Website(s)

http://mcs.open.ac.uk/jhm3/

http://www.pmtheta.com

#MTBoS Search Engine: Search posts from math bloggers.

http://www.fishing4tech.com/mtbos.html

Build graph paper

http://incompetech.com/graphpaper/

Every calculator you could possibly want

http://www.calculator.net/

A great tool for spatial reasoning

http://www.fi.uu.nl/toepassingen/02015/toepassing_wisweb.en.html

The best place to explore and create fractals

www.recursivedrawing.com

Rock, paper, scissors game that learns how you play

http://www.nytimes.com/interactive/science/rock-paper-scissors.html?_r=0

Couple with this video:

http://io9.com/use-these-tips-and-never-lose-at-rock-paper-scissors-ag-1681824570

A collection of math as represented in movies

http://www.math.harvard.edu/~knill/mathmovies/

A visual canvas to explore symmetries

http://www.mathsisfun.com/geometry/symmetry-artist.html

A list of tasks organized by topic.

http://mrkraft.wikispaces.com/Mr+Kraft%27s+Virtual+Filing+Cabinet

Tap into teen minds - a series of resources.

https://tapintoteenminds.com

Generate hyperbolic tilings.

http://www.malinc.se/m/ImageTiling.php

Interactive Scale-Balance Problems

http://solveme.edc.org/

A collection of Escher's work

http://www.mcescher.com/gallery/

A website of number talk images.

http://ntimages.weebly.com

Collection of puzzles organized by grade.

http://mathpickle.com/