yEd is a powerful desktop application that can be used to quickly and effectively generate high-quality diagrams.
Create diagrams manually, or import your external data for analysis. Our automatic layout algorithms arrange even large data sets with just the press of a button.

yEd is freely available and runs on all major platforms: Windows, Unix/Linux, and Mac OS X.

ditaa is a small command-line utility written in Java, that can convert diagrams drawn using ascii art ('drawings' that contain characters that resemble lines like | / - ), into proper bitmap graphics.

Interesting information about numbers, the Mandelbrot set, Video effects/screen savers and RIES, an inverse equation solver (you provide a solution and it generates equations).

Description of a fractal related to the Mandelbrot set and how it was rendered to a high resolution image.

Intriguing collection of examples demonstrating pictures created using PGF and TikZ tools.

PD (aka Pure Data) is a real-time graphical programming environment for audio, video, and graphical processing.

A Lisp-based interactive environment based on the Flash graphics language. To cite Kenny Tilton: This looks relatively awesome.

Xynth is a portable embedded windowing system, client/server interface between display hardware (mouse, keyboard, video displays) and the desktop environment that works on many hardware, including embedded devices (handhelds such as PSP, set-top boxes, etc.)

A free programming environment for 3D graphics from Carnegie-Mellon university.

A plotting program written in postscript, by Aubrey Jaffer. SLIB has Scheme support for it.