Opal
Opal lets you organize just about anything — ideas, reports, lists, notes, projects, graphics — in the form of an outline. If you used Acta, you’ll love Opal.
OutlineEdit
A beautifully crafted outliner for your Mac: It helps you keep your ideas structured and collected. Use OutlineEdit as an emp ...
Orgzly
Outliner for notes, tasks and to-dos
Deep Notes
Deep Notes is a simple hierarchical note pad. You can use it to outline ideas, make to-do lists, or whatever else might need j ...
OmniOutliner
OmniOutliner is a start-to-finish writing app. Perfect for collecting information, outlining Big Ideas, adding structure to an ...
Visual Outliner
Visual Outliner is an outliner software for Windows, and it uses OPML as the native file format. It's not just another tasklis ...