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Nmap is probably the most powerful network security scanner. You can use this nifty Open Source tool for security auditing, network inventory, and network monitoring. Nmap is a command line tool, but Zenmap allows you to use it with a graphical user interface. The recently released version 4.5 has more detection signatures and many more new features. Windows admins should read this first and then download the binaries here.
Always use command line if you want to master that tool.