I stumbled across these, but can't try them out yet because I'm at work. But I figured some of you would find them useful.
I think the first one used to be a commercial product but it's free now (according to the order page). The second one is open source, but it requires java and an xml input to create the meshes.
They both create high poly trees/plants, so you'll either have to reduce the polygons (milkshape3d has a poly reducer somewhere), or just use renders of what you create there as textures for treemagic or something similar.