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]
N-Stealth Security Scanner - Vulnerability-assessment product that scans web servers to identify security problems and weaknesses on local and remote web servers. Commercial and free editions are available.