Berkeley Software Design Inc. (BSD)

BSDI
Support : http://www.BSDI.COM/services/support/
Patches : http://www.BSDI.COM/services/support/patches/

FreeBSD
Security Homepage : http://www.freebsd.org/security/security.html
Patches : ftp://ftp.freebsd.org/pub/FreeBSD/CERT/index.html

NetBSD
Security Homepage : http://www.NetBSD.ORG/Security/
Patches : ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/security/patches/

OpenBSD
Security Homepage : http://www.openbsd.org/security.html
Patches : ftp://ftp.openbsd.org/pub/OpenBSD/patches/

LINUX

Red Hat
Security Advisories : http://www.redhat.com/corp/support/errata/rh61-errata-security.html
Support Centre(incl. updates) : http://www.redhat.com/support/
FTP sites : ftp://updates.redhat.com/
Kernel updates : http://www.kernelnotes.org/

Debian GNU/Linux
Security Web page (incl. fixes) : http://www.debian.org/security/

Open Linux (Caldera)
Security Advisories : http://www.calderasystems.com/support/security/
Updates : http://support.calderasystems.com/caldera?faq&15-10

S.u.S.E. Linux
Security Announcement : http://www.suse.de/en/support/security/index.html
Updates and patches : http://www.suse.de/e/patches/index.htm

Others :

SunOS/Solaris
Sun Security Bulettins : http://sunsolve.Sun.COM/pub-cgi/secBulletin.pl
Security Patches :
http://soldc.sun.com/support/index.html#patches
http://sunsolve.Sun.COM/pub-cgi/show.pl?target=patches/patch-access

SGI (Silicon Graphics)
Security homepage : http://www.sgi.com/Support/security/security.html
Patches : ftp://ftp.sgi.com/security/

IBM AIX
Security homepage : http://www.ers.ibm.com/tech-info/index.html
On-line support : http://app-01.www.ibm.com/support

Hewlett-Packard
Support Centre/Patches : http://europe-support.external.hp.com