Treepad

Description
TreePad is a fully featured interactive 3D tree modelling application for your iPad. Creating a tree is as simple as painting on the screen while watching the tree grow into your stroke. TreePad is powerful enough to capture a stunning variety of tree forms.
TreePad exports 3D models to .FBX so they can be used by all major rendering applications or viewed using Quicktime.
Best of all, its free!

here is the link for it

http://itunes.apple.com/us/app/treepad/ ... 0117?mt=8#