Hi,
It would be great if we could have an opt-in newsletter plugin :-)
Suggested features:
Opt-in checkbox when signing up for a new account
Option for non-members to sign-up with a hook to the front page capture the email address
Option to cancel subscription included at the bottom of all emails sent
Have to confirm subscription with a confirmation email
Email subscription option in users dashboard
The option to schedule send time using a cron command
Send messages in batches to minimise server load
The option to use an external mail provider such as amazon
I'm sure there would be demand for such a plugin as most popular classified sites have this feature.