Twitter's great. SMS messaging is great. Even Instant Messaging is great.
But, there's one thing which ties everyone together online. It's not going away soon, and it's something that pretty much everyone has. If you guessed email (the title of the article was a give away, wasn't it?), then you're absolutely right. Just try to imagine a world without email - even with killer audio/voice technology, it just wouldn't be as refined without email.
So, if you're seeking a solution to have Drupal email your users, then one of the best methods for doing this is the Messaging and Notifications modules. You won't find one specific Drupal email module on Drupal.org because sending email is already built into Drupal. It's they way, and when email, or any other message method is used, that you need to focus on.
Sending out a message via any of the above mentioned methods, plus a Drupal email is easily accomplished with these modules.
It's easy, because they offer all the methods plus more. One of my own recent requirements was having Drupal email all users of a given role for one specific content type, and then also support email notifications to admin users in another role.
If this is the type of solution you're seeking for your Drupal site then this video may have what you're looking for.