You should be on the security-alert mailing list of the vendor of every operating system you admin. Typically these are extremely low-volume mailing lists, and applying the security patches they recommend is frequently urgent if your machine is connected to the internet.
Apache Week, includes security alerts.
Debian: Security Announcements
Security announce mailing list for Debian Linux.
List of security announcements and information on how to sign up to security mailing list.
How to subscribe to the (free) e-mail notification service that Microsoft uses to send information about security of Microsoft products.
Security mailing list subscription.
All OpenBSD mailing lists, including security alerts.
Red Hat Linux
Notifications and advisories. Security mailing list subscription is on this page.
Last update:August 22, 2014 at 10:45:11 UTC