I like a lot this web:
Test a port using a command
$ telnet portquiz.net 80
Trying ...
Connected to portquiz.net.
Escape character is '^]'.
$ nc -v portquiz.net 80
Connection to portquiz.net 80 port [tcp/daytime] succeeded!
$ curl portquiz.net:80
Port 80 test successful!
Your IP: 213.97.29.103
$ wget -qO- portquiz.net:80
Port 80 test successful!
Your IP: 213.97.29.103
For Windows PowerShell users
PS C:> Test-NetConnection -InformationLevel detailed -ComputerName portquiz.net -Port 80