ZoomEye - Network mapping service
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Armitage - Armitage makes penetration testing easy by adding a GUI to the Metasploit framework
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
IVRE - Network recon framework, including a web interface to browse Nmap scan results.
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.