Nmap : What is it?

It is an open source security tool for network exploration, security scanning and auditing. However, nmap comes with lots of options that can make the utility more robust and difficult to follow for new users.

Its written by Gordon Lyon and it can answer the following questions easily:

What computers did you find running on the local network?
What IP addresses did you find running on the local network?
What is the operating system of your target machine?
Find out what ports are open on the machine that you just scanned?
Find out if the system is infected with malware or virus.
Search for unauthorized servers or network service on your network.
Find and remove computers which don’t meet the organization’s minimum level of security.