Put your site in maintenance mode by uploading a file.
This is a pretty simple module that checks for the presence of a file called 'maintenance.lock' in either the Drupal root directory or a site-specific directory (sites/mysite.com, for instance) and sets the site to maintenance mode if the file is present. This is very helpful for deployment scripts, such as Capistrano and the like, in which you don't always have ready access to the database, or when you want to take a lot of Drupal sites down at once without waiting for Drush to iterate over all of them.
Also, whatever text you include in your 'maintenance.lock' file will be used as the message to show users while your site is offline (you can just have an empty file too, in which case the message will revert to the default Drupal message).
Instructions for use can be found in the README.txt file that comes with the module.
This module will be deprecated as of Drupal 8.x. If I understand correctly, the Configuation Management API in D8 should allow for similar functionality. Please open an issue if this is not correct, or you have some other use case that would warrant a D8 version of this module.