If you are looking for older outliners, check out the Antique outliners section.
No two outliners are alike, but there are certain common features frequently found in outliners:
This list is based on ATPO Outliner Features 1 and ATPO Outliner Features 2, as are the initial writeups of all of the above, though it will diverge with time. Permission to use the content graciously granted by Ted Goranson; we are particularly appreciative of the diverse set of screenshots he collected. Please leave this credit note.