Looking at the screenshots reminded me of Leo. I only used it a little bit but liked it.

From their homepage:

> Leo is a PIM, IDE and outliner that accelerates the work flow of programmers, authors and web designers. Leo's unique features organize data in a revolutionary way:

> * Leo outlines are views on an underlying graph.

> * Outline nodes can reside in many places within a single outline.

> * Leo is fully scriptable in Python.

> * Leo scripts have full access to Leo's source code and all outline data.

> * Outline-oriented markup generates external files from outlines.

http://leoeditor.com/