Wireshark is a network protocol analyzer or “packet sniffer” and like all the other programs I show on this blog it’s free. A network protocol analyzer has a lot of uses, primarily education and security. You can learn a lot about network traffic and protocols from observing the packets via Wireshark. From looking at the simplest of pings, you can understand how MAC addresses, DNS, NetBIOS, and many other things all come into play.
You might also be able to detect and pinpoint security vulnerabilities/viruses/exploits by seeing frequent ARP requests or narrowing down chatter on your bandwidth. Their wiki does a good job of highlighting these sorts of tasks you can take care of utilizing Wireshark.
Download and read more about Wireshark.