Over the last month and a bit I've been working part-time on Beads, a UGen based sound synthesis and analysis library for Java. The library is under heavily development, but if you're interested in making crazy sounds and music you should definitely check it out. It is a native Java library, but can be used from within Processing without any hassles. There are a set of Beads tutorials available both for pure java and Processing (available on the website.)

Beads: "This is the website for Beads, an open-source project for computer music and sonic art in realtime performance, installation artworks, web content, and anywhere else you can think of putting it."