Tree!

Workflowy lets you organise your brain into easily manageable lists. It also works as a single-pane outliner with inline notes. It offers one-click hoisting (zooming into branches). The online version syncs with mobile apps.

Scrivener is a word processor and project management tool created specifically for writers of long texts such as novels and research papers. It won't try to tell you how to write - it just makes all the tools you have scattered around your desk available in one application.

Checkvist is a web tool for creating online outlines, hierarchical task lists, collecting and structuring all kinds of information. It has an outstanding keyboard support so you can easily re-structure your lists, filter, hoist and beef them up with tags, due dates, links, or attachments. Collaboration, Markdown, code highlighting, import and export (OPML, text) make Checkvist an ideal hub for working on complex IT projects.

Moo.do is designed to be the best and easiest way to keep your life and thoughts organized: projects, to-do lists, appointments, contacts, reminders, events, and anything else you want to write down. As all modern apps should be, Moo.do is available on any computer, tablet, or phone and synchronizes all devices and users in realtime.

germ.io is a project management tool that is centered around ideas and taking them through to execution.

UV Outliner is a small and powerful single-pane outliner. It is intended for creating, organizing, and collecting information.

OmniOutliner is a start-to-finish writing app. Perfect for collecting information, outlining Big Ideas, adding structure to any sort of writing, and much more. From grocery lists to email drafts to long-form composition.

Outliner for notes and tasks in plain-text

Tkoutline is a single pane, cross-platform outline editor written in Tcl/Tk . With this editor, information can be structured hierarchically in an outline and outlines can be hyperlinked together to create a web of outlines.

Cross-platform app for Mac, iPhone, iPad, and iPod Touch. Sync outlines between computers and devices with iCloud. Sync outlines with Evernote, Import and export outlines in OPML format.

Outspline is a free and open-source modular outliner whose functionality can be extended with addons. The most important addon is Organism, which adds advanced time management features and turns the application into a personal organizer, perfectly suited for working with todo lists, scheduling tasks and reminding events.

Fargo is a simple idea outliner, notepad, todo list, blogging tool, project organizer.

Natara Bonsai is a Palm OS and Windows Mobile outliner application with a Windows desktop client that also works as a full-featured standalone outliner and task manager. The Bonsai 5 Desktop Edition offers multiple views of a hierarchical outline: single-, dual- or triple-pane visualisations.

Buzz is a fast graphical editor for XML files with special support for OPML. Using the OPML convergence tools it will edit about any outline and many forms of indented text, including Python. In fact, Buzz was written with Buzz!

A desktop application for Windows only. Foldout is a single-page outliner, meaning the text you enter is directly organized in a tree structure. This is opposed to a dual-pane outliner, which presents a tree, but you have to click on each node to see the text within it. Supports rich text, including images. Fast and intuitive keyboard navigation. Export to text, Word, and HTML. Multiple numbering systems available (e.g. Roman numerals).