I finally got around to playing with the Arduino microcontroller the other day and started looking around for related projects. This search led me to...

Processing.org

...which is the home of the Open Source (and free) Processing programming language. The project describes it like this: Processing is an open source programming language and environment for people who want to program images, animation, and interactions. It is used by students, artists, designers, researchers, and hobbyists for learning, prototyping, and production. It is created to teach fundamentals of computer programming within a visual context and to serve as a software sketchbook and professional production tool.

The language and its integrated editing/run-time environment runs on Windows, Mac OS X, and Linux. It is a Java-based language but designed to be easy to learn and fun to use. Note that it requires the Java JDK (development kit) installed, not just the JRE (runtime environment). I installed the JDK separately. But, Processing.org provides a Windows installer that includes this for you.

Here are a couple of references about using Processing with the Arduino microcontroller:

Arduino.cc: Arduino and Processing

parchube.community: Processing/Arduino application step-by-step

Instructables: How to control LED's with Processing and Arduino