if you use open wifi networks (e.g. Starbucks, Panera, Borders, airports, etc…).
Here are some extensions, resources and articles about the hacking tool, Firesheep.
It was downloaded 104,000 times in a 24 hour period, 400,000 times in 4 days. Firesheep was released in October 2010.  Do the math.

I’ve tested the hacking tool, at Panera Bread in Spanos West, to gain access to my wife’s Facebook account on her computer as we sat there.  It works.  I could also gain access to anyone’s Facebook account there at the time, roughly a dozen, but I did not for ethical reasons.  Facebook, among many other sites, only uses authentication at the login screen.  You’ll see https in the URL bar, then navigate to the next page and you’ll see http.  Gmail uses SSL authentication throughout.  Most sites do not.  At the very least, install Firefox then the Blacksheep extension and use your smartphone’s 3G or 4G network as opposed to the free wifi.

Email me if you have any questions.

-Matt

Articles:
• Herding firesheep in New York City
• Stop Firesheep
• Lifehacker- sniff out Firesheep

install Firefox here:
• Firefox
install Firefox add-ons:
• Blacksheep
or
• Encrypt everywhere

Firesheep hacking tool:
• Firesheep
• get install

more advanced security measures:
• ssh-socks-proxy
• ssh tips