Nmap and Zenmap GUI v4.50 available
Nmap ("Network Mapper") is a free open source utility for network exploration or security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. Nmap runs on most types of computers and both console and graphical versions are available. Nmap is free and open source (license).
This is the first stable release since 4.20 (more than a year ago), and the first major release since 4.00 almost two years ago. Dozens of development releases led up to this. Major new features since 4.00 include the Zenmap cross-platform GUI, 2nd Generation OS Detection, the Nmap Scripting Engine, a rewritten host discovery system, performance optimization, advanced traceroute functionality, TCP and IP options support, and and nearly 1,500 new version detection signatures. More than 300 other improvements were made as well.