Apps to Hierarchical Tagging

Piggydb is an open-source web notebook application designed for building and managing knowledge bases, either individually or collaboratively. It emphasizes structured organization through hierarchical tagging and concept mapping.

Org-mode is a powerful and flexible mode for GNU Emacs, designed for note-taking, project planning, to-do list management, and authoring documents. It utilizes simple plain text files with intuitive markup.

ToDoList is a robust, free task management software designed for individuals and teams seeking a highly customizable, tree-structured approach to project organization. It excels in breaking down complex projects into manageable subtasks, offering features like Gantt charts, Kanban boards, and extensive export options without requiring a web connection.