I created a new drupal module called trees.
What does it do?
The trees module is based on the book module. Like the book module the trees module allows users to structure nodes in a hierarchy or outline. The main difference is that the trees module supports adding one node to multiple trees which the the book module does not. The output is also different. The book module provides a navigation, the tree module does not. Instead it produces for each tree a set of blocks, being the parents of a node, the siblings and the children.