A very controversial tool recently released called Firesheep which is a Firefox extension that allows any unskilled "hacker" to steal your session cookies for sites like Twitter, Google, Facebook, and others. These session cookies are all that a hacker needs to hijack your account.
When using an unsecured WiFi network any data sent or received is broadcast through the air in blocks of information called packets which are easily intercepted. The Firesheep tool collects this data, filters out the cookies and places them in your browser allowing you to access web pages as if you were the legitimate user. I've tested this, and it really works, so watch out! To protect yourself, use a VPN or tools like HTTPS Everywhere from the Electronic Frontier Foundation.
