Netcat is a simple Unix utility which reads and writes data across
network connections using TCP or UDP protocol.  It is designed to be a
reliable "back-end" tool that can be used directly or easily driven by
other programs and scripts.  At the same time it is a feature-rich
network debugging and exploration tool, since it can create almost any
kind of connection you would need and has several interesting built-in
capabilities.

WWW: http://nc110.sourceforge.net/