Several implementations have appeared since the Mandelbulb surfaced a couple of months ago.
The first public GPU implementation I know of was created by ‘cbuchner1′. It is based on a sample from NVIDIAs OptiX SDK, and features anaglyphic 3D, ambient occlusion, phong shading, reflection, and environment maps. It can be downloaded here (Windows only and requires a forum signup).
Example made with cbuchner1′s implementation
Very interestingly this binary runs on my laptops modest GeForce 8400M. I am a bit puzzled about this – NVIDIA state that the OptiX SDK requires a Quadro or a Tesla card, and I am not able to run the Julia OptiX demo, that cbuchner1s app is derived from.
Subblue has also created a Mandelbulb implementation, released as a Pixel Bender script and a Quartz composer plugin. A number of interesting customizations makes this my favorite choice: it is possible to explore negative and fractional powers, switch to Julia sets, and the lightning options can be fine-tuned. The only drawback is that Pixel Bender does not make it possible to directly rotate, zoom, and translate the camera – you have to rely on sliders for that.
Quilez also discovered this intimate connection between the Shroud of Turin and the Mandelbulb.
Another very interesting implementation is the GigaVoxels Mandelbulb: Whereas most implementations cast rays and use a distance estimator to speed up the ray marching, GigaVoxels use voxels stored into an Octree, which is populated on-the-fly.
For other implementations keep an eye on Fractal Forums Mandelbulb Implementation category.