Sometimes on Arqade, we use flags for moderator attention to communicate among moderators (about something specific to a question or answer).
It'd be nice if this was explicitly supported by the ability to annotate posts; I know we have chat, but this would be more asynchronous, and it could also be used to leave notes/reminders for your future self.
I don't know where in the UI I'd put it, but functionally it'd be like comments only visible to moderators.