It's often quite useful to know what running process is holding a port open. Doing this on linux has long been an easy case of running netstat with the -p option which shows the PID and name for each socket - for example:
# netstat -lnp | grep :80 tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN 14897/nginx.conf ...
But the mac version of netstat doesn't support this option. I recently came across this post on stackoverflow that points out that you can use lsof:
lsof -iTCP:xxxx | grep LISTEN
and on later versions
lsof -iTCP:xxxx -sTCP:LISTEN
Note that this also works fine on linux:
# lsof -iTCP:80 -sTCP:LISTEN COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME nginx 14897 root 48u IPv4 525163059 0t0 TCP *:http (LISTEN) ...