Lots of applications for this simple command.  Yesterday, I was troubleshooting a SQL connection issue and even though I had enabled TCP/IP in protocols, I was still unable to make a connection.  As a sanity check, I ran a shortened version of the command below which showed me that 1433 was not open and ultimately brought me to the source of the problem. 

Launch a Command Prompt as the administrator (right click, CMD, select Run As Administrator).

Enter the following command:

netstat -bano

With the additional flags I've added, this will not only show you what ports are open but it will also show you which processes are associated with the ports.  I find this to be most helpful when hunting down malware.