IMMonitor Yahoo Messenger Spy is a packet sniffer. It allows you to read what other users on your home network are sending using Yahoo Messenger. It uses WinPCap to analyse and capture all the packets that go through your network and it displays only packets that contain Yahoo Messenger chat messages.
The first thing the application does is scan your subnet to see which computers are active and it displays them all in a drop-down list. From there, you select the computer that will be the target of your spying. From the moment you start the capture of packets, messages that are detected will start appearing on the application. The packets captured by this sniffer are formatted so that you only see where the messages are from, the time and the content of the message.
All in all, this application works well, but you can only sniff one computer at a time. IMMonitor has another tool available for purchase that can monitor multiple IP addresses at once.
I personally like that all the IMMonitor Spy apps can send reports via e-mail or FTP and keep a record of all the conversations recorded. The user interface is great in all of them as well.