In PolyVox (http://www.volumesoffun.com/polyvox-about/) we provide RLE as one of the options for compressing volumes. In future versions we will also have deflate-type compression as an option. RLE is very good for 'Minecraft-style' volumes where there are few voxel values and large contiguous areas. For the mandelbulb (and medical scan output) it will be less effective since the raw data is smoothly varying