This plugin allows users to subscribe to pages, and emails them when those pages are created or changed.
It needs the Mail::Sendmail perl module, and sends mail using the local MTA.
Each user can configure which pages they are interested in, using an PageSpec on their Preferences page. Any change to a page matching the PageSpec will send an email that includes the new content of the page, and a link to the page on the web.
To make it easy to subscribe to comment threads when posting a comment, or a page, there is a check box that can be used to subscribe, without needing to manually edit the PageSpec.