QuoteResearchers from security firm Zscaler have published free software that detects when users' web connections are being monitored by a controversial tool that steals log-in credentials from Facebook, Google and dozens of other websites.
Dubbed BlackSheep, the Firefox extension alerts users when computers on a local area network are using FireSheep to steal unencrypted cookies the websites use to grant users access to their account pages. When BlackSheep detects the snoop software in a hotspot or other open Wi-Fi network, it displays a message that reads "Somebody is using FireSheep on this network." It then displays the LAN IP address of the offending party.
http://www.theregister.co.uk/2010/11/08/firesheep_detection_tool/
Could be handy, but not for me. :)