Allow the administrator to choose which groups can see private posts.
This won't fit everyone's situation, but sometimes you want visitors to be authenticated in order to read material. This will allow you to define who can view these posts.
This plugin does handle private pages, users of the appropriate level will be able to view pages that are marked as private. You may need to adjust the code in your template for your page navigation.
privatePlus has been tested with version 2.3 and 2.5 of WordPress. It may work with other versions, but I have not tested them in those systems.