Processing...

The Processing Environment is a nice one --- it's quite useful for prototyping graphical and typographical ideas.

The PDE (Processing Development Environment) is a study in simplicity and efficiency -- it abstracts much of the underlying Java-environment, and allows you to simply work with the expected graphical primitives (points, lines, rectangles, ellipses, curves, etc.), and coordinate system, using your own code to control the formation and interaction with the graphic.

I really like PDE -- it works the way you would expect: enter some code, press the run button, the result is displayed -- instant gratification. The environment supports the notion of "sketching" -- (in fact works in progress are called sketches). Want to change a color? tweak an RGB value in the code window, see the results immediately. The same applies to other areas of your graphic like sizes, primitives, loop indices, etc.

A new book on Processing has just been published: Processing: A Programming Handbook
for Visual Designers and Artists by Casey Reas and Ben Fry. The processing web site has excerpts and all the sample code from the book.