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)
...