If you're a Campaign Monitor user (an email marketing tool targeted at designers) and have wanted to dynamically add people to your lists, Vidyut Luther has a solution for you:

This is a quick how to, for people interested in adding subscribers to their Campaign Monitor lists. What you'll need: PHP 5 with SOAP built in, a Valid Campaign Monitor Account, a Campaign Monitor list.

His simple script (about 10 lines with comments removed) makes a call to the service's API and passes in your key, a list and client ID and a call to the AddSubscriber method to push the new contact into the list.