The List-Unsubscribe header is an optional chunk of text that email publishers and marketers can include in the header portion of the messages they send. Recipients don't see the header itself, they see an Unsubscribe button they can click if they would like to automatically stop future messages.
Right now the header is used to enable an Unsubscribe button in Windows Live Beta (the next version of Hotmail) and for more than a million Cloudmark toolbar users. If history is any guide, we'll see a similar feature in Outlook soon.
A List-Unsubscribe header might look like this:
Subject: [espc-tech] More info on List-Unsubscribe
Date: August 31, 2006 3:13:02 PM CDT
List-Unsubscribe: <mailto:unsubscribe-espc-tech-12345N@domain.com>, <http://email@example.com?id=12345N>
As you can see, it supports both an email based (mailto) and web based (http) unsubscribe mechanism.
The header is defined in RFC 2369 which I wrote with Grant Neufeld back in college.