I’ve been searching for a simple outline tool for writing and found this little gem, called TKOutline, which happens to be portable. This application is simple to use and is self-contained in a single exe file of 1.2 MB. It exports to text, HTML, OPML and XML.
It features the usual from outlining tools that can’t be found in a typical word processor, such as hiding levels, promoting/demoting and easily moving nodes up and down. This version of TKOutline doesn’t have an undo feature, though, so keep that in mind as you write your outline!