Some great links and utilities in this thread. I figured I would pass along a few that I know of in case any interested.

Ivy Generator is a program to make IVY grow over surfaces of an object file and then you export the ivy out to any program that will use an OBJ file ( The original object you grew the ivy on does not go with the plant. ) It does a rather nice job and I have used it to put growing ivy vines on pillars in Bryce. It does run into some issues sometimes during the import with the materials assigned, but in Bryce you can just assign your own materials and textures, so ignore the error messages on materials when importing. This is a free program and the author allows it to be distributed so I put a copy in my online briefcase for downloading. Make sure you read the text file on how to use this as some people have tried to use the program without doing this and although an easy to use program, requires a couple of steps to start the growing process.

Link:   http://briefcase.yahoo.com/bc/goshtac/lst?.dir=/3D+Free+Programs&.order=&.view=l&.src=bc&.done=http%3a//briefcase.yahoo.com/

Another handy plant program that is now free is Plant Studio for creating your own plants to import into other programs. Make sure you copy the registration info you need to enter from the download web page so you don't keep getting nagged to register. This was a commercial program at one time that the authors now give away.

Link;
http://www.kurtz-fernhout.com/download_new.html

Bruce