the best opensource (and non) advanced port scanner available. It also
a network sniffer. Without it your box is bare. It will help you a lot
tracing TCP/IP general problems and more.
if hping is the TCP/IP packets swiss army knife,
netcat is the TCP/IP connections swiss army knife.
A sniffer with a great interface and very detailed output ability.
if you want to audit some network and you can sniff the packets, hunt
is what you need. With hunt you can intruding into a connection or
sniff interesting information.