Darik’s Boot and Nuke (“DBAN”)

http://dban.sourceforge.net/
From the site:
Darik’s Boot and Nuke (“DBAN”) is a self-contained boot disk that securely wipes the hard disks of most computers. DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.
DBAN is a means of ensuring due [...]

Foundstone’s free security downloads

http://www.foundstone.com/us/resources-free-tools.asp
Free tools for security concerns from a division of McAfee. From the site:
As the premier provider of security risk management, vulnerability assessments and protection, Foundstone has created a series of tools that are used in the field by its consultants. The company offers this software free of charge to the public. Download them today to [...]

Hardware Security (and studies into the lack thereof)

http://www.security.org/
Better to be informed than blindly unaware. From the site:
Detailed information for law enforcement and security professionals regarding locks, safes, and security worldwide.

Ultimate Boot CD for Windows

http://www.ubcd4win.com/
Since Windows is usually the environment I have to help others recover their data in, or make stable again…

Select the stuff you want, and the site compiles it into a downloadable file.
Download the file and install it.
It’ll ask for your Windows directory. Let it grab data from there (assuming you have an alternative, working PC [...]

Secunia

http://secunia.com/
From the site:
Provides security advisories and information about patches, and provides software for vulnerability management.

US-CERT

http://www.uscert.gov/
From the site:
The United States Computer Emergency Readiness Team (US-CERT) is a partnership between the Department of Homeland Security and the public and private sectors. Established in 2003 to protect the nation’s Internet infrastructure, US-CERT coordinates defense against and responses to cyber attacks across the nation.

Virustotal – online virus scan

http://www.virustotal.com/
Another online scanner. Like every single virus scanner out there, it catches some, and misses others…

Internet Storm Center

http://isc.sans.org/
From the site:
The ISC was created in 2001 following the successful detection, analysis, and widespread warning of the Li0n worm. Today, the ISC provides a free analysis and warning service to thousands of Internet users and organizations, and is actively working with Internet Service Providers to fight back against the most malicious attackers.

DiamondCS

http://www.diamondcs.com.au/index.php
Like the site says, security software for Windows systems. Stumbled across them (no, not using SU) a decade ago looking for tiny apps specializing in specific security features, and these guys provided. The apps are small and very well developed.

Nirsoft.net

http://www.nirsoft.net/
NirSoft is the website of Nir Sofer, who has written a number of small, free windows utilities. These applications are useful tools that run on Windows up to and including XP, for the most part.