In my organzation, we have created a custom object that indicates a webinar someone has attended and called it a "Session". So the Session object holds all the information about the session, date, time, topic, etc, and then we associate with it all the contacts who registered for it. This is the same type of association functionality that Deals, Companies, and Tickets have.
It would be incredibly helpful if, when I click "Create Email," I'm given the option to email all of the Contacts associated with this Session object. You can imagine the scenario if a Session needs to be moved, canceled or otherwise, being able to email everyone who is registered all at once would be super helpful. As it stands, we have to create a list based on association then create a marketing or transactional email to notifiy everyone.