There are not many one-pane outliners in Windows. Sain Outliner is one of them. It's basic, but it's quite nice.
You can edit outlines quickly with keyboard shortcuts. You can number the outline headings (though there are only two numbering schemes). You can export to HTML or text files (with numbering intact). It allows you to assign basic styles to outline headings (different font and background colors). Check boxes are supported (but can be turned off). The same holds for "row lines." Files are stored in XML.
I could get used to it.
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1. See also Comparison of Outliners.