Firesheep
 

Firesheep is a firefox add-on.

It uses a packet sniffer to intercept unencrypted cookies from websites. It allows the user to instantly take on the log-in credentials of the user by double-clicking on the victim's name.

Mozilla Add-ons have stated that it would not use the browser's internal add-on and therefore they did not blacklist it.

Read more here.

This could affect Facebook and Twitter. In January 2011, facebook added more security to sessions and cookies to avoid this. (You can view it on facebook by clicking My account > account settings > account secuirty.

Although it is illegal in some countries but I think this is a real threat !