The ACL module, short for Access Control List, is an API for other modules to create lists of users and give them access to nodes. It has no UI of its own and will not do anything by itself; install this module only if some other module tells you to.

We're aware of the following modules using ACL (let us know if you know of others):

• Content Access (optionally uses ACL to provide by-user access control)
• Forum Access
• Image Gallery Access

ACL for Drupal 7
A first development snapshot is available. BEWARE: it's work in progress and completely untested! See #690618: ACL for D7 for details.

Open issues outside of ACL:

• Drupal Core 6 / Views2: A db_rewrite_sql() issue causes DISTINCT queries (frequently used in Views2) to fail, when any node access module is enabled (see #284392: db_rewrite_sql causing issues with DISTINCT). The fix was committed in #444 and appeared in 6.16. However, see #735120: Fix changes to db_distinct_field() in D6.16 / PostgreSQL and #681760: Eliminating duplicate nodes caused by node_access table joins.