Firesheep
Firesheep is an add-on designed for the Mozilla Firefox browser. This add-on allows a user to intercept unencrypted cookies from various websites (including Facebook and Twitter), effectively stealing login credentials. The user can log-in with the stolen credentials and become that user.
The use of Firesheep is a violation of computer security or wiretapping laws in some countries. Despite this, Firefox does not block the extension from running. It is possible to block Firesheep from intercepting these cookies, by using an encrypted network connection. Using HTTPS, SSL, a Virtual Private Network (VPN) connection, or Wireless Security (WEP/WPA) can accomplish this.
Also see: Cookie, Security definitions