J by Devon McCormick

Devon introduces the J programming language, highlighting both its value as a practical tool and its theoretical basis as a functional, array-oriented notation. He provides a brief overview of the entire language and demonstrate how its simple, consistent syntax, combined with its powerful primitives, gives us an unparalleled tool for developing sophisticated algorithms. He illustrates how the language works with numerous code examples.

The J language is freely available and open-source. It runs on Windows, Linux, Mac, and iOS (though this is apparently a version crippled per Apple's policies). You are encouraged to download a copy so you can follow along: http://www.jsoftware.com/stable.htm.

There is also a beta version for Android:
http://www.jsoftware.com/jwiki/System/Installation/Android - but Devon has not had much success with this, though others have.