It would be great to have automated emails for expiring membership terms - so if someone signs up in January, they get an email in December reminding them to renew and than another reminder right before renewal. The ability to customize this kind of email alert would be very helpful for membership organizations.
Auto-responses are part of our Membership feature - http://nationbuilder.com/create_membership_types#autoresponses
Click on Expiration warning to modify the email that will be sent to members on the date you defined in "Warn members before expiration" in the Settings area. By default it is set to seven days.
The expiration date of a membership is defined in the page-specific settings of the connected action page (e.g. Petition settings for a petition page).
When a membership expires, the member will receive an expired autoresponse.
If you uncheck the box next to "Automatically send this email," a member will not be notified when her membership status changes.
There isn't anything to automatically generate hardcopy letters - you could filter for "membership expires" "in the next" and put in criteria for creating a list to export for a mail merge.