SoftPerfect Network Scanner is a free multi-threaded IP, NetBIOS and SNMP scanner with a modern interface and several advanced features. It is intended for both system administrators and general users interested in computer security. The program pings computers, scans for listening TCP ports and displays which types of resources are shared on the network (including system and hidden).
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.