Outlinely Express is a free version of the Mac outliner Outlinely. (It offers the same outlining features, but has no library management.)
The program works as a text editor, and provides various ways to create lists within lists, attach notes or links to items, and collapse or expand them to hide/ remove detail.
As it is a text editor, you already know how to carry out many useful actions: change item fonts, make something bold to highlight it, copy and paste sections, search, replace, undo/ redo and more.
But there are lots of handy outlining extras. You can zoom in to an item, maybe mark it as done. Focus Mode dims everything except the current text, and items can be freely organised by dragging and dropping.
Advanced features include markdown highlighting and OPML read/ write support.
Your outline may be exported to OPML, PDF, Microsoft Word, RTF, HTML, Markdown, and plain-text formats.
A well-designed outliner, with none of the annoying restrictions you might expect from a free version.