Lsof

List Open Files (may not be installed by default)

# lsof -i tcp | grep LISTEN | sort
apache    10212 root  TCP 281.112.80.21:81 (LISTEN)
apache    15023 root  TCP 281.112.80.21:https (LISTEN)
apache    15023 root  TCP 281.112.80.21:www (LISTEN)
apache    29497 root  TCP 281.112.80.21:https (LISTEN)
apache    29497 root  TCP 281.112.80.21:www (LISTEN)
apache    29498 root  TCP 281.112.80.21:https (LISTEN)
apache    29498 root  TCP 281.112.80.21:www (LISTEN)
apache    29499 root  TCP 281.112.80.21:https (LISTEN)
apache    29499 root  TCP 281.112.80.21:www (LISTEN)
dnscache  29019 root  TCP localhost:domain (LISTEN)
master    17592 root  TCP *:smtp (LISTEN)
master    17592 root  TCP *:ssmtp (LISTEN)
smtpd      3493 root  TCP *:smtp (LISTEN)
spamd       712 root  TCP localhost:783 (LISTEN)