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TimBL has some great notes associated with his experimental Ajax SemWeb browser, Tabulator. It's an outliner, but the data underneath isn't necessarily hierarchical - the data model isn't tied to the view. There's another significant difference between this and most other outliners/browsers :

The browser browses things, not documents. Of course some thing are documents, but the browser considers them first as things, and then as documents.

The bit about directionality of links is must-read.