Currently the visibility options of a post are "Public", "Private" and "Password protected". I believe a good addition may be "unlisted" posts (can be named differently), which means that the posts is excluded by the default loops, but still accessible if one knows the URL (which differs from "private" posts in that one don't need to be logged to the site to access it). There are some ways of doing it with custom templates, yet a core option may be good for the majority of cases, in the vein of the "sticky" posts option. YouTube, Posterous, Tumblr, Google+ and other sites have similar functionality.