Yahoo Messenger Monitor Sniffer 3.0
If you want to know, what your neighbor is talking about on Yahoo! Messenger chances are you can’t. However, if you have access to the Internet gateway used by your neighbor and you can install Yahoo Messenger Monitor Sniffer there, you’ll almost surely be able to monitor his conversations. This program uses WinPcap (Windows Packet Capture Library) to sniff the network traffic. When the setup of Yahoo Messenger Monitor Sniffer finishes, you are asked to install WinPcap as well. 
The user interface is very simple. You can start or stop monitoring your network, configure which network adapter you want to use, and you can see all the messages that were intercepted in a tree hierarchy. The program is compatible with Yahoo! Messenger with Voice version 8. This is quite enough for such a program.
I tested the program on my computer, trying to see what my friends were speaking about, on Yahoo. Our computers are all connected through an Ethernet switch to another computer acting as a router. All I could do was record the conversations of users logged on my computer (I use fast user switching). My own Yahoo! Messenger conversations were captured as well. Next, I tried to install Yahoo Messenger Monitor Sniffer on the computer, which acts as a router for my local network. I was able to capture all my friend’s conversations.