Free software plug: I really like Deep Notes, a simple outlining app. When working on longer writing, I need to see the structure in outline form while being able to quickly dig down into that structure and look at the exact wording.
Deep Notes represents hierarchical structure with expand/condense buttons, like list view in the OS X Finder, giving complete control over the level of detail you see for every item. It’s easy to move items around in the hierarchy; it’s a dynamic model, which is exactly what I need when sorting out conceptual structures.
If anyone has seen this functionality in a web app, please let me know!