Tidbits turned 18 recently. The celebratory email referenced article series almost as old as Tidbits: TidBITS: Matt Neuburg - Conquer Your Text 1993 to 2006.
The classics are there: Symantec MORE, Inspiration (still sort of living), Acta, In Control, Arrange and WebArranger, (Double) Helix, Prograph, Idea Keeper, Boswell, StickyBrain, Tinderbox, iData Pro, Notebook (both of them), DEVONthink, TAO, Curio, Yojimbo, SlipBox and a few others.
There was terrific software in that collection, some of which is still sold. On the other hand the history is a reminder of the terrible cost of proprietary data formats; many of those apps took user data with them when they expired.
I love this class of software, but I wouldn't personally consider anything with a proprietary file format.