Link 1: Snap! Build Your Own Blocks. Beta

Link 2: SNAP! Reference Manual [55 page PDF]

Snap! (formerly BYOB) is a visual, drag-and-drop programming language. It is an extended reimplementation of Scratch (a project of the Lifelong Kindergarten Group at the MIT Media Lab) that allows you to Build Your Own Blocks. It also features first class lists, first class procedures, and continuations. These added capabilities make it suitable for a serious introduction to computer science for high school or college students.

SNAP! runs in your browser. It is implemented using Javascript, which is designed to limit the ability of browser-based software to affect your computer, so it's safe to run even other people's projects, even if you don't trust our competence or good intentions...