Get on the same WiFi as your target, open up Wireshark and grab their HTTP communications.
To make this easier, there was/is a tool called Firesheep that can be used to hijack session cookies. The popularity of Firesheep caused many sites to enable HTTPS by default (e.g. Facebook did so).
Comments
How do you "steal" a cookie?
The best bet is generally an xss attack. Though there are other ways, you could sniff one on a wireless network if no encryption is in use.
Get on the same WiFi as your target, open up Wireshark and grab their HTTP communications.
To make this easier, there was/is a tool called Firesheep that can be used to hijack session cookies. The popularity of Firesheep caused many sites to enable HTTPS by default (e.g. Facebook did so).
If you need to be on the same WiFi as your target I really don't see the big problem, realistically speaking.
Common, shared wired LANs at offices and workplaces are a problem. Home LANs, where family members need privacy from one another, is also a problem.