If you use Firefox:

A recent addon for Firefox called FireSheep allowed anyone using it access to your web accounts via session hijacking IF you were using a non-password-protected internet connection, such as a McDonald’s, Starbucks or even an airplane.

The only way around that was to use a VPN, but now there’s another addon from Zscaler Cloud Security called BlackSheep, which warns you if anyone on your network is using FireSheep.

Here’s how BlackSheep works: Firesheep’s packet sniffing can’t be detected, but what can be detected is Firesheep’s requests to websites like Facebook using your cookies. BlackSheep detects this type of activity by making requests to random sites known to FireSheep every five minutes (you can adjust the timing) with fake values.

If anyone else on the network starts using those same fake values to make requests, then BlackSheep knows someone on the network is using Firesheep, and you get a warning in your current browser tab.

BlackSheep was based on Firesheep’s code to ensure its effectiveness. In a word, for every hack, there is an equal and opposite counter-hack.

Mashable

Be safe out there!