NewGrowth is a Blender add-on that lets you draw trees interactively. Using brush strokes you can direct the growth of twigs and let them grow into branches. You can prune existing branches as well. NewGrowth gives you full control to create unique trees with real character.
Once a tree is created many aspects of the generated mesh and leaf particle system are parameterized, like the overall thickness of the trunk or the amount of tiny twigs that are pruned, giving you control over your creation even after you have saved the result.
NewGrowth is designed for intuitive drawing and comes with on-screen help as well as a comprehensive manual.