The lsof (list open files) command will list open files. The -i option limits it to open network connections:

$ lsof -i 
COMMAND    PID   USER   FD   TYPE DEVICE SIZE/OFF NODE 
    NAME

firefox-b 2261 slynux   78u  IPv4  63729      0t0  TCP 
    localhost:47797->localhost:42486 (ESTABLISHED)

firefox-b 2261 slynux   80u  IPv4  68270      0t0  TCP 
    slynux-laptop.local:41204->192.168.0.2:3128 (CLOSE_WAIT)

firefox-b 2261 slynux   82u  IPv4  68195      0t0  TCP 
    slynux-laptop.local:41197->192.168.0.2:3128 (ESTABLISHED)

ssh       3570 slynux    3u  IPv6  30025      0t0  TCP 
    localhost:39263->localhost:ssh (ESTABLISHED)

ssh       3836 slynux    3u  IPv4  43431      0t0  TCP 
    slynux-laptop.local:40414->boney.mt.org:422 (ESTABLISHED)

GoogleTal 4022 slynux   12u  IPv4  55370      0t0  TCP 
    localhost:42486 (LISTEN)

GoogleTal 4022 slynux   13u  IPv4  55379      0t0  TCP 
    localhost:42486->localhost:32955 (ESTABLISHED)

Each entry in the output of lsof corresponds to a service with an active network port. The last column of output consists of lines similar to this:

laptop.local:41197->192.168.0.2:3128 

In this output, laptop.local:41197 corresponds to the localhost and 192.168.0.2:3128 corresponds to the remote host. 41197 is the port used on the current machine, and 3128 is the port to which the service connects at the remote host.

To list the opened ports from the current machine, use the following command:

$ lsof -i | grep ":[0-9a-z]+->" -o | grep "[0-9a-z]+" -o  | sort | uniq