del.icio.us has a neat feature where if you tag a URL with for:carlfish, that URL will end up in my inbox. Inbox labels are private, in the sense that only the target username can access the summary page or RSS feed for that label.

This would be a neat way to bring pages to someone's attention. For example, if I want Mike to edit some documentation, I could label it as for:mike, and it would end up in his "inbox", which would presumably be accessible on the labels page of his profile. (Maybe we need some kind of "new" indicator to say that there's new stuff in the inbox?)