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 [...]

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.

Virustotal – online virus scan

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

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.

Knoppix

http://www.knoppix.org/
The home site for documentation and download links to Knoppix, a Linux distribution, live CD/DVD, and system recovery suite able to scan Windows systems and retrieve data, if not fully repair the system.