At this page you can read about port scanning and see a collection of free port scanner software and port viewer software.
Read detailed information about Port Scanning at Articles
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.
Read more about NMap>>
Angry IP scanner is a very fast IP scanner for Windows.
The program itself is small and it can scan IPs in any range.
Angry IP scanner first pings each IP address to see if it's live, then resolves host names, scans ports, etc. as you have configured the scan.
It also has additional features, like NetBIOS information (computer name, workgroup name, currently logged in user and MAC address)
and it can save results data as CSV, TXT, HTML, XML or IP-Port list files.
Read more about Angry IP Scanner >>
LanSpy is network security scanner that allows you to gather a variety of different information about computer:
Domain and NetBios names, MAC address, Server information, Domain and Domain controller information,
Remote control, Time, Discs, Transports, Users, Global and local users groups, Policy settings, Shared resources,
Sessions, Open files, Services, Registry and Event log information.
Read more about LanSpy >>
Read more about Free Port Scanner software
You may find that you have ports open (e.g. by using the scanning software above) but that doesn't tell you exactly what's going on.
That's where local software to view what ports are open can come in handy,
particularly when it can show what application or process is using each port.
Viewing the process attached to a port is for the most part only supported using NT/2000/XP.
Read more about Free Port Viewer software
