The best to date
Ive used quite a few outliners over the last 3 decades on dozens of systems, and although there have been better ones on systems that no longer exist (ie Amiga, DOS, etc), this is currently my favourite. Simple, easy to use, does what I expect it to, and above all, no extra fluff or feature bloat. Also has a good number of export/import options. If you need something to organize and outline your thoughts, this is the way to go!
Fainswift about
Outliner, v3.5.1