Firesheep

Firesheep is an add-on planned for the Mozilla Firefox browser. This add-on allows a user to point unencrypted cookies from various websites (including Facebook and Twitter), effectively stealing login credentials. The soul can log in near the stolen credentials and become that user.

Source: http://feedproxy.google.com/~r/LatestNewPagesOnComputerHope/~3/_91vzQgdlhA/firesheep.htm