Nessus: Security scanner for Linux, BSD, Solaris, and other flavors of Unix. Performs over 900 remote security checks, and suggests solutions for security problems

Nmap: Developers of NMAP, a network port scanner and service detector offering stealth SYN scan, ping sweep, FTP bounce, UDP scan and operating system fingerprinting. Downloads, documentation, security tools directory and a mailing lists archive. [Windows, MacOS, Unix

The Cross Site Scripting FAQ: Answers questions on identification, threats, and prevention. Provides examples and links