A plug-in or theme that would allow the site owner to create page or posts which are restricted to certain members or even non-members.
The idea would be that the wordpress user could post content and even links to files that are intended for one person or company.
That person or company ("visitor") would have a login name and password to be able to access their content.
When the visitor logs in, they are shown a listing of the pages / posts which they have access to.
When a new page/post is created by the site admin or user with permission, they would select which person / companies could have access AND the people selected would be sent an email notifying them of the content's availability.