Yeah, well, it doesn't do much good in 2009 if you are on Windows (or if you have to use Windows at work). I used ThinkTank and More all the time in the 1980s (yes, I used a Mac back then - and I still do now at home at times). Now, I'd love to use an outliner like OmniOutliner (true outliner with multiple columns), but there isn't anything I can find (by the way, I want it as a desktop app, not on the web, sorry). It's a shame because my brain thinks in hierarchies a lot. It seems so strange that there are many good outliners for Mac, but none for Windows. Why is that?
Dave Winer's later creation, Frontier, has been open sourced: http://frontierkernel.org/ It works, but I wouldn't try to edit code with it. Other than its built-in language, which is actually sort of fun, but a dead-end technology now. (Still, being a language built for an outliner has interesting consequences.)
I've been working on a Frontier-descended outliner for a long time (really just the outliner, not the whole stack that Frontier represents), but up until recently, open-source toolkits couldn't do a slick outliner as the text widgets were too weak. Now I lack time to finish it, but I'm still cranking away in my spare time. (To crap something out is easy; to make a useful outliner is a harder problem that it appears.)
MS Word has an outlining mode, and since it dominates the Windows ecosystem, there's very little economic incentive to build a world-class dedicated outliner.
I use TreePadLite[1] on Windows (and JreePad[2] everywhere else) whenever I need to make a simple hierarchical document to serve as input to a template engine or code generator. It's not open source but the interface is intuitive and the document format is trivial to parse and transform.
