Qtodo is a hierarchical TODO-list manager based on Qt4 and ruby. Version 0.1 is pretty usable, although documentation is rather inexistant (just right-click, and you'll have some ideas of what is doing what).

It features three simultaneous views:
* a tree view showing the hierarchy of elements
* a flat list view where sorting is possible
* a (simple) text editor to display and edit notes related to the current item

Data is saved using YAML, and is very easy to parse and modify outside of QTodo.