The Notify module has been ported, but it deliberately only handles a few basic common use cases. I'm wondering if other people have run into the need for more complex solutions, and/or have thoughts about what would be best to try to migrate.
There's Subscriptions, Notifications (http://drupal.org/project/notifications - the editor here is not letting me create links), Message Stack (https://www.drupal.org/node/2180145), several others - a comparison of features is at https://groups.drupal.org/node/15928
From the numbers given there, Message Stack is the most well established. On the other hand, Subscriptions is just one module and does much of what I need to do.