Back in the summer, I worked on an outline of the Book. This work was made quite easy thanks one app bundled with MacOS - OmniOutliner 2. Too bad the tool is payware and doesn't run on Linux.
So, how do I do outlines in Linux? XEmacs is my favourite text editor, but I'm sorry to say, I hate its outline mode. Kind of like using giant spoon to fling paint on the wall: Does the job, but isn't probably the best tool for it, and your fingers will get a funny new colour too.
I tried Zim, but after a good consideration it seems to have a rather wrong... paradigm for my needs. It's more of a note wiki than an outliner. I have a MediaWiki install around, thank you very much. =)
Today's big try was gjots2. Good news: Somewhat sane file format. Can output HTML. Pretty lightweight. Bad news: Debian package is borked due to Python being in transition, so I had to rely on Stow; Don't know what's wrong, but the app seems curiously unstable (hit a key at a wrong time, and the app crashes). But otherwise, it's good, and I like the UI more than I like OmniOutliner's, really (one big edit box). The only lacking thing from my point of view is text styling.
There's this thing called MindRaider, I wonder what it's worth; I couldn't understand the UI at first. Got to see how it works!
Small update: I tried MindRaider. Seems good, but it keeps its repository in one place, has a little bit stiff UI, and it can't export stuff to different formats. And, annoyingly, it complains that I have Java 1.5 installed. Markup features are good though. And as odd as it may sound like, I think I've picked the Emacs outliner - got to just bite bullet and learn the darn thing. At least writing scripts that dump the things to HTML is dead simple...