Role Scoper plugin by kevinB is a comprehensive access control solution, giving you CMS-like control of reading and editing permissions. Assign restrictions and roles to specific pages, posts or categories. For WP 2.7 to 2.9, use Role Scoper 1.2.x.
WordPress core role definitions remain unchanged, and continue to function as default permissions. User access is altered only as you expand it by assigning content-specific roles, or reduce it by setting content-specific restrictions.
Users of any level can be elevated to read or edit content of your choice. Restricted content can be withheld from users lacking a content-specific role, regardless of their WP role. Deactivation or removal of Role Scoper will return each user to their standard WordPress access (but all RS settings remain harmlessly in the database in case you change your mind).
Scoped role restrictions and assignments are reflected in every aspect of the WordPress interface, from front end content and navigation to administrative post and comment totals. Although Role Scoper provides extreme flexibility and powerful bulk administration forms, basic usage is just a set of user checkboxes in the Post/Page Edit Form.
Partial Feature List
WP roles work as is or can be limited by content-specific Restrictions
RS roles grant additional Read or Edit access for specific Pages, Posts or Categories
Define User Groups and give them one or more RS roles
Can elevate Subscribers to edit desired content (ensures safe failure mode)
Control which categories users can post to
Control which pages users can associate sub-pages to
Specify element(s) in Edit Form to withhold from non-Editors
Grant Read or Edit access for a limited time duration
Limit the post/page publish dates which a role assignment applies to
Customizable Hidden Content Teaser (or hide posts/pages completely)
RSS Feed Filter with HTTP authentication option
File Attachment filter blocks direct URL requests if user can’t read corresponding post/page
Inheritance of Restrictions and Roles to sub-categories / sub-pages
Default Restrictions and Roles for new content
Un-editable posts are excluded from the Edit Posts/Pages list
Optimized to limit additional database queries
Integrates with the Revisionary plugin for moderated revisioning of published content.
Supports custom Post Types and Taxonomies (when defined using WP schema by a plugin such as Custom Post Type UI)
Extensive WP-mu support
Abstract architecture and API allow other plugins to define their own data/taxonomy schema and role definitions
Author provides some extensions to support integration with other plugins
Theme code can utilize the is_restricted_rs() and is_teaser_rs() functions to customize front-end styling.
Other useful functions include users_who_can(), which accounts for all content-specific roles and restrictions.
There is a support forum and useee guide at: http://agapetry.net/downloads/RoleScoper_UsageGuide.htm