SharePoint Stramit Document Library Browser is a free package of 3 Treeview WebPart for the SharePoint document library.
I have always complained about the lack of navigation options that are in the the document library feature of SharePoint.
Lots of people speak about migrating all shared documents from a WSS site.
Great idea, but the user loses some of their common tools such as the NT Explorer. Browsing a document hierarchy always remembers an Explorer view and a Treeview.