Firesheep

After installing the extension you’ll see a new sidebar. Connect to any busy open wifi network and click the big “Start Capturing” button. Then wait. As soon as anyone on the network visits an insecure website known to Firesheep, their name and photo will be displayed. Double-click on someone, and you’re instantly logged in as them. That’s it.
Gotta try this out.
Source: daringfireball.net