WP-Flock is a plugin that provides LiveJournal-like custom security groups for posts and pages. It’s more flexible than Post Levels, and less complicated than Role Scoper. Plus it hooks into JournalPress; what more could you want?
It is currently in its “stable beta” stage, and as such some features may not be available or a little wonky.
The latest updates about the plug-ins development can be found in the project blog.