Advisories and Patches

Debian Bug Tracking System - Automatic system tracking bugs reported by users until they are resolved.
www.debian.org/Bugs/

Debian: Security Information - Details of current and archive security advisories. Includes mailing list and a security FAQ.
www.debian.org/security/

Linux Security Patches - Security Patches and upgrades
www.linux-sec.net/Patches/

Red Hat Errata - Errata for all releases of Red Hat Linux.
www.redhat.com/support/errata/