1. Say you have a blog website and your blog has 5 categories. Then you have a subscribe registration form where people can subscribe to one or more of the existing categories. Also, when a new category is created, the registration form will list it. Then you create a script that each time a new post in a certain category is posted, people subscribed to that category get an automatic email showing the title and excerpt of the post and a button to click to read the rest on the website.

2. A subscribe to newsletter form, allowing sign up with Google+ and Facebook.