netcat

Like cat but it uses a socket for stdin/stdout
Use "nc host port" to connect to a server
- -u will send data via UDP (but you have to have something to send)
- -z will connect and then quit (useful for scanning TCP ports)
Use "nc -l -p port" to listen for a connection
- -u is UDP again
- You can listen for particular IP addresses
Available at http://www.atstake.com/