Drupal has built in access control for content types but we need finer control.
The first use is to solve the following problem. All blog entries (a content type) are viewable by all in the default system but we don't really want our site development blog entries cluttering up vistors view of the site. Access has been denied to all but administrators to view blog entries tagged with sitedev.
Taxonomy access module
Access control for user roles based on taxonomy categories (vocabulary, terms).