TracAnnouncer is a flexible trac notifications drop in replacement that is
very flexible and customizable.  There is a focus on users being able to
configure what notifications they would like and relieving the sysadmin
from having to manage notifications.

HACKING NOTES:

The most confusing part of announce is that subscriptions have three
related fields, that are not intuitive.  (sid, authenticated, address).
There is a very good reason for this.  First, Trac users are identified
throughout the system with the sid, authenticated pair.  Anonymous user
are allowed to set their sid to anything that they would like via the
advanced preferences in the preferences section of the site.  The can
set their sid to the same sid as some authenticated user.  The way we
tell the difference between the two identical sids is the authenticated
flag.  There is a third type of user when we are talking about announcements.
Users can enter any email address in some ticket fields, like CC.  These
subscriptions are not associated with any sid.  So the sid and authenticated
in the subscription would be None, None.  These users should be treated
with all default configuration and permissions checked against anonymous.
I hope this helps, because it took me a while to wrap my head around :P