trn
The threaded successor to rn, which let you follow a Usenet discussion as a conversation rather than a pile of loose letters.
Around 1990 Wayne Davison took Larry Wall's rn and bolted threading onto it — the name means "threaded read news". The reader tracked which post answered which via the References header and drew the thread's shape as a small character-art tree in the corner of the screen, so you could see exactly where in the argument you were. The commands were essentially rn's, which made switching painless.
Through the first half of the 1990s trn was the default reader on university Unix boxes, usually paired with a well-tended killfile. It read and wrote the same .newsrc as its predecessors and fetched articles over NNTP. The competition was tin and later slrn, both friendlier to newcomers, but trn users had already paid the learning cost and saw no reason to pay it twice.