Some lazy registration systems capture email addresses without forcing the user to create a password. That's what ours does.

http://www.90percentofeverything.com/2009/03/16/signup-forms...

[edit] We have an email alert feature on our job board platform, that allows job-seekers to get email notifications for new job ads. Standard functionality. When we removed the registration barrier from this user journey (so it's now just a single text field and submit button), email alert creation rates basically doubled on most client sites.

http://labsblog.madgex.com/2010/10/the-impact-of-email-activ...