Members

Manage users, roles, and capabilities. Manage content access/permissions of users by their roles.
Basic features:

  • Role Manager: Allows you to edit, create, and delete roles as well as capabilities for these roles.
  • Content Permissions: Gives you control over which users (by role) have access to post content.
  • Shortcodes: Shortcodes to control who has access to content.
  • Widgets: A login form widget and users widget to show in your widget areas.

Learn More

WordPress Plugin Directory: http://wordpress.org/plugins/members/
 
An in-depth guide is included in the plugin download. (It's the readme.html file in the plugin's /docs folder).