Max/MSP/Jitter isn’t for the faint-hearted, but those who can get their heads round the complex diagrams swear that the procedural, node-based approach is immensely powerful. There’s a huge community of developers too, producing patches for all sorts of activities. It’s expensive, though. This is the world of boxes – draw lines between components, set values for inputs and outputs, wrap them up into new boxes, provide more connections for inputs and outputs, possibly even using recursion, and you can achieve pretty well anything.

Isadora is a cheaper alternative. I don’t know much about it, but I get the impression it’s largely used in dance, presumably because it was developed by a dancer.

For Apple Mac users, there’s the little gem called Quartz Composer that comes free with every copy of the operating system, though it’s not installed by default. It, too, is procedural, and I gather that the latest version for Snow Leopard (OS 10.6) is even better.

Then there are programs such as vvvv and Visual Jockey, both of which are free and Windows only as far as I know.

Coming full circle, back to Processing, we have Field. I know nothing about Field apart from what’s in this article, but it looks very interesting.