How to Scan for Open ports in your computer or any other machine

There are plenty of ways to find out which ports are open in your machine or any other machine in your LAN or on the internet.Its very important to keep a track of what all ports are open on your system as any unused open port could be very dangerous!!
One of the easiest ways to look for open ports in any Operating system is by going to THIS LINK .By default the ip address selected would be your own, if you would like to scan ports of some other pc, then you have to enter the ip of that machine.
Ubuntu by default has a very cool port scanner.It is available Under System->Administration->Network Tools. Once you go there you should see a page similar to this.

Now click On the Port Scan tab . If you want to scan your own PC then enter Network Address as 127.0.0.1 otherwise enter the ip address of the machine you want to scan ports.And then Hit the Scan Button and you should find a list of open ports in the column below that.