While Post-it Plus allows you to categorize notes, NoteRush takes this concept a bit further. It is pitched as "the best desktop app for creating linked notes and to do lists." I have not extensively tested, but it "is a Windows desktop application that helps you to write notes and build your information in a highly interactive and engaging way. NoteRush is centered around post it style note windows that can be linked and grouped as much as you like into dynamic floating collections."
There are two versions: Classic (which is free) and Premium (which costs 29.95). It needs .NET Framework 4.
The interface takes some getting used to, but NoteRush does what it promises to do and works like a vertical outliner of sorts.
The free version does not allow you to pin notes or export them. As I have not bought the application (or even actvated the 20 day trial), I could not test these features. From the little I have use NoteRush, I have developed a feeling that the problem in using the application for the long run will be that its interaction with other applications is a bit clunky. This may, of course, be wrong, and I will report on this later.
Overall, it seems like a good idea.