Searching today for a free-form outliner on Windows, I found wikidPad. It’s a private wiki implemented as a Windows application. It augments the classic anything-goes wiki model with per-page attributes that aggregate up into structured views, creating a neat blending of the two worlds. As a bonus, the whole thing is implemented in Python (though it is not open source), and there are rudimentary hooks for extending it.
A similar product for the Mac is VoodooPad, though I’ve never used it, so I don’t know what other things it does, or how similar they are.