From X-Plane Wiki
Blender is a free (under GPL-License released) 3D-Modelling Tool. With Blender you can create 3D-Objects, texture and animate them. Blender supports Python as Scrippting Language.
With the XPLane2Blender Plugin, you have the ability to export the 3D-Model you've created with Blender, in the X-Plane OBJ-format.Since Version 2.5x of Blender you cannot import *.obj-Files anymore.
XPlane2Blender is hosted at github.com.