I used Processing as the starting point for a recent project ( http://www.tinkthank.net/software/tiction ). Prototyping with Processing and its accompanying IDE got me up and running literally within minutes.
Inevitably I exported the whole thing to an Eclipse project because I needed more flexibility with threading and things, but for prototyping anything with a strong visual element I couldn't imagine a more perfect framework.
Processing in Javascript: http://processingjs.org/exhibition
A related browser-based project with a simpler syntax, Algorithm Ink: http://www.azarask.in/blog/post/contextfreejs-algorithm-ink-...