This is my first WordPress plugin in a while. Its a modification of the Force User Login plugin that extends it in two ways:
1) It works with WordPress multisite, so that you can set on a per site basis if a site requires login to view the content.
2) It allows you to set a minium user level to view content, so for example, only Editors and above can view the content. (This is useful when you have automatic user registration set up or on a multisite where users may have access to multiple sites.)
Get it from WordPress here