Date Reminder allows a registered user to request an email reminder to be sent at a specified time before an event. Generally these would be calendar events, but any node type that has a CCK datetime can support reminders.
Features
When reminders are enabled for a node, registered users (with the appropriate privilege) can request reminders to be sent before each occurrence of the date/time. The user specifies how long before the date/time to send the reminder.
When reminders are enabled for a node type, anyone who can edit the node can enable or disable reminders for that node.
Reminders are sent using the Messaging system. The usual reminder method is email, but other messaging transports should work. (Well, not Drupal 7. The Drupal 7 version just uses straight email.)
System administrator can control:
Which node types allow reminders, and which date field (if there are several) in a type to use as the basis of reminders.
Which (registered) users can request reminders.
Which users can request reminders to arbitrary email addresses instead of their own.
Which users can see or administer other users' reminders.
Content of reminder messages.
If and how long reminders should be kept after last date occurrence.
The list of allowed lead times for reminders from which users can select.
An effort is under way to move much of the documentation here to the Date Reminder section of the Site Builders Guide. Currently that document is very incomplete, but watch there. Information on this module page will gradually be moving there.