The best open source alternative to ZBrush is Blender. If that doesn't suit you, our users have ranked more than 50 alternatives to ZBrush and ten of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to ZBrush are Bforartists, Armory 3D, SculptGL and BlenderCompat.
A comprehensive open-source 3D software suite for modeling, UV unwrapping, texturing, rigging, and animating. It supports renderings, simulations including fluid and cloth dynamics, complex compositing, video editing, and Python scripting, running across all major operating systems.
Bforartists is a complete 3D suite to create 3D content. It offers you the full 3D art pipeline to create game graphics, prerendered movies and stills.
Armory is an open-source 3D game engine with full Blender integration, turning it into a complete game development tool. The result is a unified workflow from start to finish, making you work faster. No more jumping between different applications.
Compatibility DLL for running Blender 3.x on Windows 7 and a patch to modify the Blender source to use it.
CrackArt is a free open source 3D paint and texturing application that includes Texture Painting, Texture Baking, Procedural Materials and Textures, and much more.
A game engine written in Python, extending support across multiple GPU devices with 3D rendering, PBR Renderer and much more.
Dilay is an open source, easy to use 3D sculpting application. With an update, roughly, one every 2 weeks, it is well maintained.
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it's free , more simple and has many other uses but it may not be as powerful as ZBrush in terms of performance or sculpting