Working in Splinex LLC software company, I propose and initiated developing VoxelArt - voxel editor.
The first version was made in Borland Delphi and looks promising, so it was decided to make commercial project (using Borland C++).

The team of developers and designers was made for this. But, to the time the project was released, I’m not working in Splinex anymore. It has no commercial success. We just don’t know how to use such graphics (but later Minecraft shown how it could be, indeed!) 

Anyway, I love voxels, and today we use voxels widely - for example, for rendering clouds in Unreal Engine in realtime (see Night sleep was taken from my eyes project).

Idea was to use moving plane in 3D for editing objects.

You can download VoxelArt demo, but it’s for Windows XP and crashes at start.
Luckily, you could download VoxelArt help file (as PDF) too and explore how it looks like (Figs. 2-5).

1. https://sourceforge.net/projects/perevalov-tech/files/2004/VoxelArt/

2. https://cloud.mail.ru/public/9REG/aLZM3M66h

(Get from archive

The idea was patented (USA Patent): Method and apparatus for rendering, storing and editing voxel objects
Publication number: 20050151734
Publication Date: Jul 14, 2005
Inventors: Andrew Gubkin, Denis Perevalov, Dmitry Philonenko , Valery Sharin, Peter Novak, Peter Zakrevsky

Fig. 1.  Object in VoxelArt editor


Fig. 2. Beginning of VoxelArt Help, explaining what is voxels

Fig. 3. Creating new voxel space dialog in  VoxelArt

Fig. 4. Part of VoxelArt Help, explaining voxels editing idea

Fig. 5. Part of VoxelArt Help, explaining Clay Pen and Eraser brushes