tcprelay

Programs like tcpdump, Ethereal and the like make it difficult to simply log
traffic exchanged over TCP between two end points. This program simply
performs a forward between a server and a client and logs it.

The primary purpose is to log the traffic, so as to see what is going on
through an SMTP connection for example.
In addition, as it relays traffic, it allows to do "pass-through" networking
on top of TCP.

The logging of the traffic is done in binary format by default, you can also
get telnet-style logging, assuming the traffic is made of printable lines
(suitable for traffic like SMTP, POP3, etc.)

It provides similar functionalities to tcptunnel, of Clemens Fuchslocher,
available here:
http://www.vakuumverpackt.de/tcptunnel/