Blender - Blender is the open source, cross platform suite of tools for 3D creation.
SpeedTree - SpeedTree is a powerful toolkit used to create 3D vegetation for video games, films, and animations.
Autodesk 3DS Max - 3ds Max is software for 3D modeling, animation, rendering, and visualization. Create stunning game enrivonments, design visualizations, and virtual reality experiences.
The Grove 3D - Model or grow adaptive 3D Tree models for use in CG renderings.
ZBrush - ZBrush is a digital sculpting and painting software solution.
Tree Studio - Easy 2D Tree Creation