SOSTX

one needs to be aware of what needs fixing/updating/patching - from operating system to apps to plugins in those apps.  An excellent resource is Secunia.
For daily update:
http://secunia.com/advisories/historic/

For a typical CentOS 5.5 server with Joomla, Moodle, Webmin the following links are worthy of bookmarking and checking on a regular basis:

Operating System:
http://lists.centos.org/pipermail/centos-announce/
(versions 4 and 5 of CentOS)

Applications
(note that most are about plugins/add-ons to the core apps)

Joomla

http://secunia.com/advisories/search/?search=joomla&sort_by=date

Has an Admin plugin for updating core - many good/quality 3rd party addons now have update buttons in the Admin of the add-on.

Moodle

http://secunia.com/advisories/search/?search=moodle&sort_by=date

Has CVS updates via command line

Word Press

http://secunia.com/advisories/search/?search=wordpress&sort_by=date

Now has CVS updates via command line and newest version has updating on the Admin side

Webmin

http://secunia.com/advisories/search/?search=webmin&sort_by=date

Update notifications within Webmin itself