Wireshark is the world’s most popular network protocol analyzer. It is used for troubleshooting, analysis, development, and education.

Not sure I have a need for this but it good to know its around.

Link:
http://www.wireshark.org/

Check out these posts too:

  1. The ABCs of securing your wireless network
  2. theBroken – How to hack a wireless network
  3. How to Crack a Wi-Fi Network's WEP Password with BackTrack
  4. Very powerful free network monitor
  5. Network Monitor 3 One Click Capture Tool
  6. Monitor your network (professionaly)
  7. Create Your Own Social Network with Ning