Software
WebGL
See the dedicated WebGL section.
Max for Live
Max is a programming environment with an interactive audiovisual focus. Ableton Live is software for music creation and performance. Max for Live combines them, letting you extend and customize the functionality of Live with Max.
Read more on the Max for Live pages.
Processing
"Processing is an open source programming language and environment for people who want to program images, animation, and interactions.... It was created to teach fundamentals of computer programming within a visual context and to serve as a software sketchbook and professional production tool."
Learn more and download Processing @ the official Processing site.
A few things I've made with Processing: