Nmap, a simple Network Mapper, is a powerful port scanner tool. This free and open source hacking tool is the most popular port scanning tool around that allows you to easily perform network discovery and security auditing. Used for a wide range of services, Nmap uses raw IP packets to determine the hosts available on a network, their services along with details, operating systems used by hosts, the type of firewall used, and other information.
Nmap is available for all major platforms including Windows, Linux, and OS X.
We have written about how you can use this simple tool to find SQL Server instances on your network.