Nmap - Network Security Audit Tool
Nmap is an open source tool for network discovery and security auditing. Many systems and network administrators also use this tool for network inventory, managing service upgrade schedules, and monitoring host or service uptime.
It uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. It was designed to rapidly scan large networks but works fine against single hosts.
Nmap suite contains a results viewer (Zenmap), a flexible data transfer, redirection, and debugging tool (Ncat), a utility for comparing scan results (Ndiff), and a packet generation and response analysis tool (Nping).
It was featured in the following movies: