netstat

Displays network-related information such as open connections, open socket ports, etc.

List all ports:

netstat -a

List all listening ports:

netstat -l

List listening TCP ports:

netstat -t

Display PID and program names:

netstat -p

List information continuously:

netstat -c

List routes and do not resolve IP to hostname:

netstat -rn

List listening TCP and UDP ports (+ user and process if you're root):

netstat -lepunt

Print the routing table:

netstat -nr