The authors of django-treenode describe it as "probably the best
abstract model / admin for your tree based stuff".

Features:
- Fast - get ancestors, children, descendants, parent, root, siblings, tree
  with no queries
- Synced - in-memory model instances are automatically updated
- Compatibility - you can easily add treenode to existing projects
- No dependencies
- Easy configuration - just extend the abstract model / model-admin
- Admin integration - great tree visualization: accordion, breadcrumbs or
  indentation