TerraForge3D is an open source actively developed procedural 3D modelling tool for creating terrain and other 3d objects. They just released a fairly major update adding Linux support, a new PBR based renderer and more. You can create 3D terrain using CPU or GPU based generator or create your own using the node based tools.
Details of the update:
Shader Node Editor
A blender like node editor for shaders.
A new and advanced way for baking and exporting Textures.
Added Linux Support
Now TerraForge3D is supported natively on linux.
Linux Version is in Alpha
Now TerraForge3D has a Physically Based Renderer for high quality preview
New Modules API
Now you can extend TerraForge3D however you want and write custom plugins using the powerful TerraForge3D SDK.
Not a big thing from users point of view but for anyone wanting to develop TerraForge3D its a huge deal. The codebase has been massively cleaned up.
You can learn more about TerraForge 3D and see in it action in the video below.