Nmap (“Network Mapper”) is a free and open source (license) utility for network exploration or security auditing. Many systems and network administrators also find it useful for tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime.
Net Scan Tools
NetScanTools is a collection of over 40 network utilities for Windows, designed with an easy user interface in mind. It includes DNS tools, a ping and port scanner, traceroute, and other utilities.
SuperScan is a powerful TCP port scanner that includes a variety of additional networking tools like ping, traceroute, HTTP HEAD, WHOIS and more. It uses multi-threaded and asynchronous techniques resulting in extremely fast and versatile scanning.
Angry IP Scanner
Angry IP Scanner (or simply ipscan) is an open-source and cross-platform network scanner designed to be fast and simple to use. It scans IP addresses and ports as well as has many other features.
It is widely used by network administrators and just curious users around the world, including large and small enterprises, banks, and government agencies.