Tkoutline
Tkoutline is a single pane, cross-platform outline editor written in Tcl/Tk .
20 Alternatives To Tkoutline
Altova XMLSpy
The XMLSpy XML Editor is a powerful tool for editing XML and related technologies. It is the only XML Editor with patented SmartFix validation, enterprise-grade editors, converters, debuggers, and code generators.
Joplin
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
OmniOutliner
OmniOutliner is a writing app for collecting information, outlining ideas, adding structure to any sort of writing.
OneNote
Get the OneNote app for free on your tablet, phone, and computer, so you can capture your ideas and to-do lists in one place wherever you are. Or try OneNote with Office for free.
Simplenote
The simplest way to keep notes. Light, clean, and free. Simplenote is now available for iOS, Android, Mac, and the web.
Visual Outliner
Visual Outliner is an outliner software for Windows, and it uses OPML as native file format.