This plugin allows you to choose more than one category as a private category. Only users with a certain level will be able to see posts in the private categories.
Both the private categories and the user level needed to see private categories are configured from an options panel.
Two small changes need to be made to single.php and sidebar.php for full functionality.
http://willwyatt.com/blog/?page_id=19 for the plugin
This is my first plugin, I'd love some feedback about how I did.