Windows only: TreeSheets is a hierarchical notes manager structured like a spreadsheet. Although it sounds unwieldy, it's surprisingly easy to move around and create well structured notes, to-do lists, and more.
At first glance TreeSheets looks like a spreadsheet application. The structure is distinctly the grid and cell organisation familiar to anyone who has ever wrangled an Excel document. The cells in TreeSheet however are designed with text in mind and the application handles large amounts of data within each cell rather effectively. Using keyboard shortcuts you can easily nest cells within each other or create an outline-like structure complete with headers and subsets. TreeSheets can used entirely with keyboard shortcuts which makes manipulating the data extra speedy. The first document that opens upon initial run is a tutorial to help get you started, additional information about using the application be found in the online tutorial. If you're not sold on the spreadsheet interface, share your favourite note taking tool in the comments below. TreeSheets is freeware, Windows only.