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.

The Visualization ToolKit (VTK) is an open source, freely available software system for 3D computer graphics, image processing, and visualization used by thousands of researchers and developers around the world. VTK consists of a C++ class library, and se