Here’s an interesting video that shows hackers sniffing the Wifi to retrieve Gmail cookie. Of course, this is when you are using Gmail without HTTPS secure access.
For secure Gmail, simply use https://gmail.com instead of http://gmail.com.
For analyzing your network whether that be Wifi or wired, you might want to consider trying the following TCP/UDP network analyzers.
Download Ethereal – I have used Ethereal for a long time before when I used to work as an embedded network engineer. It’s a great tool and shows pretty much every packet your network passes around. (great for wifi)
Download Wireshark – Similar to Ethereal, I believe this tool is based on the same core as the Ethereal.