Processing is a flexible software sketchbook and a language for learning how to code within the context of the visual arts.
Javascript implementation of processing, "reinterpreted for today's web"
openFrameworks is an open source C++ toolkit for creative coding
SuperCollider is a platform for audio synthesis and algorithmic composition, used by musicians, artists, and researchers working with sound.
Cinder is a free and open source library for professional-quality creative coding in C++.