Admin_only_mode is a project mainly written in PHP, it's free.
Drupal module providing an admin-only mode
This module provides a less-disruptive alternative to placing your site in full maintenance mode.
It allows you to place your site into an admin-only mode, so that only the user with uid 1 is able to login. All other users are logged out automatically and cannot login again until the site leaves admin-only mode.
Anonymous users are unaffected by this mode. If you need to stop anonymous users from posting comments or other actions, you should consider the readonlymode or mode modules instead of or in addition to this module.
To use this module, simply install it and either:
Many thanks to the readonlymode module for the coding examples.
-Ben Claar ben at claar DOT net