Voraldo grew out of some experiments in the space of pixel-sized voxels. I think this is extremely interesting stuff, and it has gone through roughly 13 iterations, over a period of several years. Some of the early experiments were very slow, CPU based offline rendering processes. The more recent ones provide realtime 3D interaction using a custom alpha blending process running in a compute shader.
The basic idea is to be able to manipulate the contents of a 3D texture, including the alpha channel, and see the changes in realtime. It also does some interesting things with volumetric lighting, in this same space. It has also proved to be a cool testbed for messing with medical volume data. You can find out more in the articles below: