Org-mode is for keeping notes, maintaining ToDo lists, doing project planning, and authoring with a fast and effective plain-text system, and it’s FREE.
Org-mode is an add-on for the popular free EMACS editor (programmers editor) and adds a full outline and todo list. All the files are stored in standard txt (ASCII) format so can be edited by any software if required. If you have a dropbox account you can store your outlines on your dropbox account and edit them using any editor or use the EMACS add-on to see the proper outlining.
The author suggests that this wonderful outline tool can be used for GTD, planning, notebooks and web/pdf authoring. There is also extensive documentation. There are some good tutorials covering introduction and power users guides with a useful screencast overview (25 minutes).
The beauty of this product is that it is very extensible and can be adapted by power users to do just about anything that requires outlining and maintaining lists. The files can be published to HTML and XHTML plus other formats such as LaTeX and Gantt Charts for project management integration. There’s a very interesting tech talk from GOOGLE that discusses Org-mode (46 minutes) and well worth listening to.