The ACL module, short for Access Control Lists, 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)
The refresh module adds the functionality to have the "node display page" automatically refresh (using a meta refresh) using a (per node) configurable number of seconds.
RoleAssign specifically allows site administrators to further delegate the task of managing user's roles while withholding the Administer permissions permission.
This module allows users to set privacy options for their profile fields when using the core profile.module. It does not create any settings pages or configuration screens, and it integrates seamlessly with the existing profile module.