Subscribers can now keep per-user notes, which show up whenever you view a post by that person.
H/T to SJS for the suggestion. I think others have suggested something similar in the past; all I saw on the ideas site was for user notes visible on the ignores page, though.
UPDATE: snowbird asked for it way back in 2015:
https://www.cougarboard.com/board/message.html?id=14424618