Firesheep

Firesheep is a really cool extension that scans the wireless network you are on and gets cookies. and from those cookies it enables you to access the unencrypted parts of the website (from what I've read the only parts that usually are encrypted are the username/email and password). To install it all you have to do is download the file and open it in firefox and then download it as you would any other extension.

NOTE: I DO NOT CONDONE OR ENCOURAGE USING THIS TO HACK PEOPLES ACCOUNTS SUCH AS FACEBOOK OR TWITTER.

Update: this only works on unencrypted networks not on networks with WEP, WPA, or WPA2.