Subscribers-only plugin lets you mark posts for subscribers-only. You can mark all posts in a category as subscriber-only, or mark individual posts regardless of category, or both. You define which roles are allowed to read subscriber-only posts. No template modifications are required.

When a post is marked as subscriber-only and your home page displays excerpts then the excerpt will be shown along with an optional “Subscribers-only” banner that you define. When a reader clicks on the link to go to the individual post page they must be logged in to read the post. If a person is not logged in then they will be presented a link to login or register.

Pricing: Free
Requirements: Wordpress 2.0+
Source: http://edwards.org/2006/03/07/subscribers-only-plugin…