I like to help others learn. Having to teach mathematics in graduate school taught me that. I won the B.F. Bryant Teaching Award at Vanderbilt many years ago, and I have always been proud of that.
I like to build things that help me learn. It's nice when I can tell that something I built has helped someone else learn as well.
Javascript is a wonderful way to accomplish all of these things. I have used it to build a number of interactive visualizations within the last year that have allowed me to understand things in a way I don't think I could have otherwise:
 A statistical simulation of births and deaths realtime in the U.S.
 A realtime searchable tweet map of the U.S.
 A realtime searchable vine map of the U.S.
 A Box2D visualization of money in real time piling up based on a specified rate
 An interactive visualization to help understand how the Pagerank Algorithm works
 An interactive bigram analyzer for arbitrary documents
 A Bayesian Bandit interactive visualization
 An svg marker explorer (it's tricky to understand how the various parameters affect these things)
 A cooccurrence matrix explorer for understanding how a simple recommendation system works

An online powerball simulation to give you a feel for how long it will probably take to win the powerball, and how much you will spend.
Even for what are really "simple" things, like the "Idea Muscle Exerciser" I did last Sunday afternoon, I learn something new about javascript, or css, or a specific browser, or a deeper appreciation of the subject of the visualization itself. It has always been satisfying.
This is just scratching the surface of what is possible for education with this medium, and of course I'm not the only one doing it. The visualizations that I've done so far are in somewhat specialized and advanced niches based on my own interests, but these same techniques can be used for elementary and secondary educational tools. For example, in linear algebra, interactive visualizations that can help folks learn for once and for all how matrix multiplication works. How eigenvalues and eigenvectors work. Solving systems of equations.
There are certain things that a lot of folks get stuck on when it comes to certain aspects of mathematics. Having a simple interactive visualization that they can bang on, can watch as the interacting pieces of the process are highlighted in the proper context at the right time, can be incredibly effective.
These are exciting and satisfying times.