Impromptu : Live Coding

Impromptu is a live coding environment utilizing Scheme(a dialect of Lisp) and is a fantastic environment for prototyping.  Developed by Andrew Sorensen, how Impromptu access and controls your Mac is beautiful and elegant.  

And before discovering the advances in Javascript and HTML5, it was my favorite technical sandbox.  Here's a small OpenGL program I wrote while learning matrices to implement ArcBall rotation: