This is a great stand-alone tool that was first mentioned by "sonix" in a thread on blenderartists.org.
The tools developer, Thomas Luft, writes:
A few month ago I was looking for a new sample scene to test watercolor renderings. I was thinking of something complex, filled with vegetation - like trees overgrown with ivy. Fortunately I was able to implement a procedural system so that the ivy would grow by itself. The result is a small tool allowing a virtual ivy to grow in your 3d world.
The ivy grows from one single root following different forces: a primary growth direction (the weighted average of previous growth directions), a random influence, an adhesion force towards other objects, an up-vector imitating the phototropy of plants, and finally gravity. This simple scheme reveals that the goal was not to provide a biological simulation of growing ivy but rather a simple approach to producing complex and convincing vegetation that adapts to an existing scene. The ivy generator imports and exports obj+mtl files.
The application was originally written for the Windows platform but, with Mr. Luft being generous enough to also provide the source code, it has been ported to the MacOS platform and the Linux platform (thanks to Ruben), as well.
The software and its source code is released under the GNU General Public License. Also, the developer had originally restricted the usage of the application:
Please respect that it is NOT PERMITTED to use this software or parts of it in commercial projects!
However, after a few inquiries were made by several persons, Mr. Luft has been quite generous, yet again, and has now removed the commercial usage restrictions!
Awesome Tool! Thanks, Thomas Luft!
Note: Be sure to read the "readme.txt" file for the instructions. Some additional Blender-specific tips, provided by "sonix", can be found at the blenderartists link. Thanks "sonix"!