It ws a few years ago that TreeGenerator 1 came out. version 1.2 is still available for free.

version 2 is out now with na new 3D interface (OpenGL). Still a few things that need improvements, but it already creates interesting results for trees and generates low-polygon versions with billboards. Could be useful for gamedev.

Check details here:

http://www.thebest3d.com/treegenerator