List-Unsubscribe is a mail message header used to provide machine readable unsubscribe instructions - an email address, a URL or both.

If List-Unsubscribe is present, some mail clients will offer a unsubscribe button and/or unsubscribe option if a user tries to flag the message as spam.

It is a recommended 'best practice' to improve message deliverability and do the right thing for the end user.

Documentation:
http://www.faqs.org/rfcs/rfc2369.html
http://www.list-unsubscribe.com/

Gmail support: https://mail.google.com/support/bin/answer.py?answer=81126#unsub
http://techcrunch.com/2009/07/23/gmail-tries-to-make-it-easier-to-unsubscribe-f rom-spam-newsletters-but-fails/

Microsoft support:
http://download.microsoft.com/download/e/3/3/e3397e7c-17a6-497d-9693-78f80be272 fb/enhance_deliver.pdf
(although may be broken?):
http://blog.deliverability.com/2011/04/has-hotmail-discarded-the-list-unsubscri be-functionality.html