Orgzly - Outliner for notes, tasks and to-dos
Tkoutline - Tkoutline is a single pane, cross-platform outline editor written in Tcl/Tk .
MemPad - MemPad is a plain text outliner and note taking program with a structured index.
Workflowy - A better way to organize your mind.
Foldout - The single-pane outliner that Windows users have been searching for for years, free!
OutlineEdit - A beautifully crafted outliner for your Mac: