With the full release of Unity 5, you can have AAA tree editing tools by subscribing to the SpeedTree Modeler.
Note: You must have Unity 5 Beta (not alpha) to use SpeedTree with the engine. Trees and plants created with the SpeedTree Modeler will not work with Unity 4 or any other pre-5 Unity release, and will not work with Unity 5 alpha.
Highlights of SpeedTree for Unity 5 Beta:
Scalable wind effects, including Wind Wizard
Seamless LOD tuning
Create unlimited variations with one click
Branch seam blending
Sub-division surface modeling
Add collision primitives automatically or manually
Modeling with mesh forces (e.g. grow trees around or through 3D meshes)