On my website for STE I am using a Join module with the text "Subscribe", allowing non-members to join the site and therefore be signed up for the PM-based newsletter to all members.
If you are not a member yet (none of the Wikidot team are), you can see it in action here: http://www.wdeditor.com/
Similarly, it would be useful to provide an "unsubscribe" link to members - functioning in the complete opposite way to the Join module. (hidden from everyone except members, and it removes a member from the site).
As this is unlikely to be used as often as the Join module (people want others to join their sites, not leave them!), it's probably best as an extension to the Join module rather than a completely new module.
In fact, I think that the only people that would use this are people that use the Join module in the same way I am - as a 'subscription' link for the website's newsletter. If anyone has any other ideas, please post them below and rate the wish up ;-)
