Savory's Notification Service
This awesome service can handle all the notices coming out of your application, whether by email, stored in a MongoDB "inbox," or via your own pluggable delivery mechanism.
To handle very large scales, it works by first having you queue new notices (in MongoDB), and then it sends them asynchronously (via /tasks/ configured in crontab) to the adressees. Notifications sent to millions of users can be queued just once, on a "channel," which individual users can "subscribe" to. It's essentially a subscription system, though it also handles individually sent notices, to known users or even to on-the-fly addressees.
Refer to the Savory.Notification API documentation for more details.
Digests
Still working on documenting this feature, though it's otherwise pretty complete!

