Cross-platform
Processing [Cross-platform]
Computer programming language and IDE for visual arts.
https://processing.org/
Cinder [Cross-platform]
Open source library for professional-quality creative coding in C++.
https://libcinder.org/
openFrameworks [Cross-platform]
Open source C++ toolkit for creative coding.
http://openframeworks.cc/
Unity [Mac, Win]
Game engine, but useful for creative coding and installations.
https://unity3d.com/
Open-source creative coding framework for iOS.