[UPDATE] TreeLine v3.1.0February 21, 2019
TreeLine is a cross between a database and the outliner. It stores data in a tree structure, but every node in the tree serves like a database using its own data types and field formats. It supports a huge selection of import/export options, among the most used being for Mozilla-type bookmarks.
Note: This application is not a typical tree-based outliner and demands some effort to digest and read in order to grasp the the program’s data model and related theories. Watch the How to Use part of the website.