SpeedTree - SpeedTree is a powerful toolkit used to create 3D vegetation for video games, films, and animations.
RealWorld Paint - RealWorld Paint is an free portable image/photo/animation editor.
Tree Studio - Easy 2D Tree Creation
miniPaint - Online graphics editing tool use HTML5 technologies like Canvas, Offline support, Drag and Drop.
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
JugiPaint - JugiPaint is a painting software with diverse tools for various types of illustration work.