JOE is an outliner written in Java. (Thanks, Pete.) It’s open source (”infinitely-long free trial”). It can read plain text and OPML files (although not malformed OPML files). Prebuilt Windows and Mac binaries are available.
It’s interesting, but it’s not my cup of tea, any more than any other outliner. I’ll stick with the code-folding ActivePython for Python coding, and Emacs for everything else. I suspect that everyone who wants an outliner either already has one or has figured out how to make their favorite text editor behave like one, and the rest just don’t care.
