The tcpsound utility plays sounds in response to network traffic, making it possible for a user to literally listen to a network. It forks a pseudo terminal in which to run tcpdump, parses that output, and plays a wide variety of user-configurable sounds. By interpreting the output in a pseudo terminal, users can first SSH to a remote host if desired.
tcptrack is a packet sniffer, which passively watches for connections on a specified network interface, tracks their states, and lists them in a manner similar to the Unix 'top' command. It displays source and destination addresses and ports, connection state, idle time, and bandwidth usage.
tcpxd is a TCP/IP relay or proxy, allowing a connection to a port on a system to be forwarded to another port on any other system. It is useful for firewalls and service relocations, and is small, simple, and fast. It's also easier to learn than netcat and supports TCP_NODELAY, alternate local ports, partial connection closures, and non-blocking connects.