Some of the really fancy email systems out there like Infusionsoft, Marketo, Aweber and others let you do what is known as a targeted drip campaign. This really just means that you can send specific messages to a group of your list based on what they do or don't in your emails.
For example, if your reader clicks on a link that is in your email, you could send a follow up email based on that click. Make sense?
You could send an email announcing two specials on your website for example. Standard email as you've done in the past. Let's say that the email includes Link A, and Link B.
Let's also talk about two readers, hypothetically Jane and Joe. If Joe clicks on link A, he can be put into a queue to receive emails after that click. These emails could be specially crafted for people that click on link A.
At the same time, Jane clicks on link B. This action puts her into a different queue with different emails.
This is a marketer's dream because you can easily send different emails based on the actions of your reader. The same can be done for people that don't click on any links. This is a really nice addition to Mailchimp. You can read about this and other changes in their blog.
They've updated the way auto-responders work, the features available and other stuff worth taking a look.