The Views Mail module provides a flexible method for sending messages to lists of users created by the Views module. You can view the history of how this module came about here: http://drupal.org/node/134931
The 6.x version is currently actively supported. The 5.x version will no longer be maintained.
The .dev versions previous to November 2009 work with versions of Simplenews prior to RC6.
The .dev versions from November 2009 work with versions of Simplenews from 6.x-1.0.