Outliner software
The golden age of the outliner software has been in the 80's and the 90's where dedicated software was widely available, especially on the Mac OS platform. Nowadays, the outlining concept has been integrated in an important number of software product (like file explorers, or the libreoffice suite), completing their core functions set. As a result there is little dedicated outliner software left, with the exception of the mobiles platforms where this kind of tools seems to regains interest from users (especially on iOS).