There's a semi-active discussion of outliners at http://www.memes.net that has references to Frontier and MORE and a few other interesting tidbits. The site itself is interesting, in fact conceptually addicting.

(BTW and somewhat OT...I wonder how many of us really appreciate the value of being able to type a URL like above and have the underlying software recognize it as one and create a link automatically. Every time I use a piece of UserLand software, I get that same kind of feeling.)