SpeedTree - SpeedTree is a powerful toolkit used to create 3D vegetation for video games, films, and animations.
Unity - The multiplatform game creation tools for everyone.
The Grove 3D - Model or grow adaptive 3D Tree models for use in CG renderings.
Godot Engine - Feature-packed 2D and 3D open source game engine.
Tree Studio - Easy 2D Tree Creation
Blender - Blender is the open source, cross platform suite of tools for 3D creation.