I am using Keynote NF (Open Source Tabbed notebook with RichText editor and multi-level notes)
KeyNote NF is a flexible, multi-featured tabbed notebook, based on Windows standard RichEdit control. Its always accessible with a single keypress, even if you work in another application.
The basic idea in KeyNote NF is that you can include many separate notes within a single file. This means that you do not need to open several files - for most purposes it is enough to create only one file and hold all your notes inside it. With the addition of the tree-type notes, you now have a three-dimensional notebook: many notes within one file and a multi-level, nested pages within a single note. Keynotes interface and behavior are extremely configurable.
With powerful text formatting capabilities, easily navigable interface and additional features such as styles, macros, plugins, and templates, KeyNote is has become the favorite note keeper, diary, outliner, knowledge base and information manager for thousands of users (including myself)