Firesheep is free and open source and works on Mac OS X and Windows with Linux support being promised soon. The download rate of this add-on is epic with over 320,000 downloads in 3 days. The tool is incredibly easy to use, log in to a public WiFi spot and click a button you’ll then be shown images and user names of various people using networks such as Facebook, Twitter, Flickr, bit.ly, Google and Amazon. With a double click on their image, you’ll be logged in as them immediately.
Source: https://thehackernews.com/2010/10/firesheep-social-network-session.html