Nmap
Open SourceNmap is a free and open-source utility for network discovery and security auditing. It utilizes raw IP packets 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 is command-line driven, though official GUI versions are also available.
