Subscriptions
This module enables users to subscribe to be notified of changes to nodes or taxonomies, such as new comments in specific forums, or additions to some category of blog. Once enabled, all nodes will have an additional link that allows the user to change their subscriptions. Users have tab on their user screen to manage their own subscriptions. Users can also set an auto subscribe function which notifies the user if anyone comments on posts they have made. Admins can set this on by default.
Subscriptions has its roots in 2003 (node/4189!). At version 5.x-1.x it was in a crisis, due to too many people having pulled it in too many direction. In Winter '07 and Spring '08 it was rewritten from the ground up by chx and salvis, and these two are maintaining it to this day.
A lot of work and testing has gone into the new version for Drupal 6, and we now have a very flexible suite of modules that can be tuned by the administrator to fit a large number of use cases.
Subscriptions is a modular framework, designed to be extensible in many directions. Developer documentation is available at api.drupal-contrib.org/api/subscriptions, created by develCuy, sponsored by www.transit.york.ac.uk. Also look inside one of the component or add-on modules that has functions that you're interested in.
Drupal 6.14: A core bug was committed in #193383-106: set_time_limit: Centralize calls and prevent warnings and errors that keeps Subscriptions from sending out notifications. Please update to Subscriptions 1.1 which has a work-around for this core issue. Other contributed modules that run on cron are similarly affected.
