Messaging Framework allows messages to be sent in a channel independent way. It provides a common API for message composition and sending while allowing plugins for multiple messaging methods.
When using this framework, you won't send e-mails to users anymore. You will send them 'messages' and they will decide how they want to get them delivered, that may be by mail, IM, SMS, depending on user's preferences.
Releases & issue tracking
http://www.drupal.org/project/messaging