Firesheep
codebutler.com/firesheep
What Jamie wrote
Fascinating, frightening free Firefox extension from Eric Butler lets you hijack (sidejack, to be technical) accounts on popular services like Facebook and Twitter by capturing unencrypted session cookies sent over the local network.
It's extremely common for websites to protect your password by encrypting the initial login, but surprisingly uncommon for websites to encrypt everything else. This leaves the cookie (and the user) vulnerable. HTTP session hijacking (sometimes called "sidejacking") is when an attacker gets a hold of a user's cookie, allowing them to do anything the user can do on a particular website. On an open wireless network, cookies are basically shouted through the air, making these attacks extremely easy.