It is inspired by Adobe's Pixel Bender, but uses GLSL, and is created specifically with fractals and generative systems in mind.

Features:

Multi-tabbed IDE, with GLSL syntax highlighting
User widgets to manipulate parameter settings
High-resolution (tile) renderings
Progressive rendering, for high quality AA, and distributed ray tracing.
HDR (RGBE .HDR) support and tone mapping
Support for simple animations
Modular GLSL programming - include other fragments
More than 100 examples, including 3D fractals (Mandelbulb, Mandelbox, Quaternion, Kaleidoscopic IFS), 2D fractals (Mandelbrots, Ducks, spherical Julias), Heightmaps, Knots and Hyperbolic Tesselations.
Fragmentarium is built in C++, OpenGL/GLSL, and Qt 4.