One of the coolest scripts that came along with Blender 2.46, is the Trees From Curves. This script was developed for the Peach Project, to create trees and brushes. If you want to know the basics of how this script works, Ideasman42 has produced a video tutorial, showing the basics to create a tree model.
The link for the video is available at the Blender Wiki, together with the documentation of the script. So, go there and start to make your own trees!