Nmap

Nmap (Network Mapper) is a free open source tool for computer availability and security auditing. It scans large networks very quickly.

Many system and network administrators find this tool useful for tasks such as network inventory, upgrade plans, host monitoring or service uptime.

Nmap uses raw IP packets in novel ways to determine which hosts are available on the network, what services (application name and version) hosts offer, what operating systems (and operating system versions) are running, what type of packet filters/firewalls are in place, and dozens of other characteristics.