An outliner is built in to many word processors and there are a few standalone outliners also available for Mac users.
Outliners can be used for research projects, story writing, capturing notes on a per-subject basis, planning, and much more. Most outliners, Mac or Windows, have one thing in common. The vertical expandable collapsible outline. Here’s a Mac outliner that dares to Think Different™. It does outlines, yes, It works much like any outliner you’ve ever used, yes.
Tree is an outliner that displays information with a horizontally expandable tree view.
Take a look.
That may seem like a crazy feature (Tree also features a standard vertical outline view), but it’s not. On wide screen Macs the horizontal outline view gives you a different perspective, especially when using Tree for planning.
Also worthwhile is Tree’s ability to open multiple documents in different tabs.
The tabs bar gives you quick, one-click access to multiple outlines. Also built-in to Tree are Checkboxes for to-do lists, and Notes to add quick notes to items.
Files can be stored in multiple text formats, too, including Plain Text, Rich Text, Word, and OPML documents.
As an outliner, Tree is more full featured than expected, and the addition of a horizontal view is a big plus. That said, we live in an ever changing, dynamic world these days, and Tree definitely needs an iPad counterpart with files that sync on iCloud or Dropbox.