BranchList is a todo list that allows you to create subtasks as deep as you want to. Each item can become a new list and you can go to one level down as many times as you want.
The UI is intuitive and allows you to drag down or pinch apart to create a new item. You can go to the level above by pulling all the way down, or go down a level by clicking on the # of item a tasks has.
You can also take notes on the app the same way!
Data is backed up on Google Drive, Dropbox, or Box. Just login with your account.