I just spent way too much time looking for this. Treeline is very close, but it doesn't allow marking a task as done with a single key press. It's a powerful data organizer and one has to change manually defined data types to achieve that.
I'm looking for something simple: a list of tasks that can be organized into a tree that allows adding children, siblings and moving of items and marking them as done (ideally through keyboard shortcuts).
Best Answer
Take a look at tudu. It's a simple ncurses based hierarchical task manager. It's in the Ubuntu universe repo.
You can add children, move in or out of different levels of the tree hierarchy, and mark tasks as done. Everything is single key commands. It's fast, straightforward, and lightweight.