@frownland Ran into this post as a link from the post about Jef Raskin, and reading it I suddenly realized: you can do the same kind of thing as Lotus Agenda, using DEVONthink. It has named "smart groups" which can pick out items based on tags, content, filename, annotations, and so on.

Somehow this had never occurred to me even though I've been using DEVONthink for over a decade and used to work for Lotus.