Adobe
- Security bulletins and technical briefs that provide information to customers.
- http://www.adobe.com/support/security/

HP: IT Resource Center
- Americas/Asia-Pacific site offering online support. Includes knowledge base, patches, drivers and security mailing lists.
- http://www2.itrc.hp.com/

SGI: IRIX
- Security advisories and patch downloads. Includes mailing list.
- http://www.sgi.com/support/security/

Microsoft
- Download security updates, service packs, and anti-spyware tools for Windows.
- http://www.microsoft.com/downloads/search.aspx?displaylang=en&categoryid=7

Cisco
- Patches and advisories for Cisco products
- http://www.cisco.com/en/US/products/products_security_advisories_listing.html

Red Hat Linux Security Resource Center
- Alerts, whitepapers, articles, presentations, and tools.
- http://www.redhat.com/security/resources/

HP: Tru64 Unix
- Security advisories and associated patches.
- http://h30097.www3.hp.com/unix/security-download.html

Compaq
- Patches and advisories for most Compaq products.
- http://www1.itrc.hp.com/service/home/compaqMain.do

Red Hat Linux Errata
- Security advisories and updates (patches) for all supported versions.
- http://www.redhat.com/security/

Netscape
- Security alerts for netscape client and server software.
- http://wp.netscape.com/security/index.html

SCO Unix System Security Enhancement Area
- Advisories and patches. Note this is an FTP site.
- ftp://ftp.sco.com/pub/security/sse/

Apache Web Server
- Advisories and patches.
- http://www.apacheweek.com/security/

Citrix
- Security bulletins and hotfixes for Citrix products
- http://support.citrix.com/latestsecurityall!execute.jspa

Oracle
- Patches and other security information.
- http://otn.oracle.com/deploy/security/alerts.htm

SCO
- Security information for Caldera OpenLinux, OpenServer, OpenUnix, and UnixWare.
- http://www.sco.com/support/security/

NetBSD
- Advisories, patches, links.
- http://www.netbsd.org/Security/index.html

FreeBSD
- Advisories, mailing lists, tips, tricks, guidelines, links.
- http://www.freebsd.org/security/

OpenBSD Security
- Advisories and information.
- http://www.openbsd.org/security.html

SuSE Linux
- Patches directories for all supported versions, plus a security announcements list (English).
- http://www.suse.de/en/support/download/updates/

Sun Microsystems
- Security and recommended patches for Solaris: patch clusters and individual patches.
- http://sunsolve.sun.com/pub-cgi/show.pl?target=patches/patch-access

KDE
- Only if you've built KDE yourself; otherwise probably available from your linux distribution vendor's patches directory. Note this is an FTP site.
- ftp://ftp.kde.org/pub/kde/security_patches/

OpenBSD Release Errata
- Advisories and patches.
- http://www.openbsd.org/errata.html

HP-UX Patch Additional Information
- Patch Security Matrix, list of obsolete patches, patch checksums.
- ftp://us-ffs.external.hp.com/export/patches/

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

AIX Patches - Fixdist
- You need the "fixdist" program to install AIX patches.
- ftp://aix.boulder.ibm.com/aix/tools/fixdist/

AIX
- Security and other patches for AIX. Note that this is an FTP site.
- ftp://aix.boulder.ibm.com/aix/fixes/

FreeBSD Patches FTP Site
- Security advisories and patches.
- ftp://ftp.freebsd.org/pub/FreeBSD/CERT/

Microsoft Security
- Microsoft-product-related security headlines and bulletins.
- http://www.microsoft.com/security/