A useful combination of options for the #netstat command on #linux is "-n -p -u -t -l -e": show all listening udp and tcp sockets, include program PID and extra information, do not translate numbers.
If you speak german, these options are easy to remember in this order: netstat -tulpen -- what means "tulips" 馃尫. A single tulip, netstat -tulpe turns on number resolution.
#FreeBSD equivalent is sockstat -l -Ptcp,udp