Frontier has been reborn. Dave Winer's outline editor is now GPLed and runs in a browser: http://scripting.com/2013/09/16/concordOurGplOutliner

There's a forked version that can save to the local filesystem here: https://github.com/interstar/OWL

This needs to be upvoted to the top, as this is the best possible answer to OP's question — with an open-source implementation!

I wonder if this good be a Getting things Done tool. You'd probably need multiple of those trees and a way to put simple, wiki-like multiline articles (maybe markdown syntax?). I'd use something like that!

You can do that with Fargo.io. Concord out of the box appears to just hold a single outline.