OmniOutliner Pro is an old OS X app -- been around forever. I wouldn't be surprised to learn it started out on the NeXT workstation.
I've owned it for years, but I've never quite fit it into my workflow. The main value for me has been that it does a terrific job of opening my ancient Symantec MORE 3.1 outlines. That's handy, since 10.5 orphaned many of my old papers.
Years ago I thought I'd use it to create web pages, but it did a lousy job -- back then. Happily the OMNI Group keeps improving it, and they've never asked me for more money. A recent update mentioned something about 'dynamic html outlines', so I gave it a try.
Impressive! You can create very nice dynamic HTML outlines. Since OmniOutliner can open RTF documents it's not a bad way to put some documents on a web site -- assuming you have FTP access.
I might get some new use from my old app.