Android users might want to check out MindNote on Google Play. It's presented as a "mind mapping notepad", but it's really a simple, single-pane outliner. You can easily indent or outdent entries as well as easily move them up or down the hierarchy. It has a hoist function called "focus". It will also export and import plain .txt files. It has checkboxes in the left margin so it will also function as a no-frills ToDo or list app. It has unlimited hierarchy levels. No fancy formatting but seems to be very functional outliner.