Nmap ("Network Mapper") is a free open source utility for network exploration
or security auditing.  It was designed to rapidly scan large networks,
although it works fine against single hosts.  Nmap uses raw IP packets in
novel ways to determine what hosts are available on the network, what services
(application name and version) they are offering, what operating system (and
OS version) they are running, what type of packet filters/firewalls are in
use, and dozens of other characteristics.