A few days back, a new Firefox add-on called Firesheep was developed which lets anyone with the add-on installed capture other users' cookies over WiFi. It essentially meant that anyone on the network can hack into anyone's Facebook, Twitter etc. whenever they login to it over the network. Scary stuff indeed!
Now, Gunnar Atli Sigurdsson from the University of Iceland has developed a program called FireShepherd to counter Firesheep. A name like FireWolf or something like that would have been more awesome though afterall what the program does is kill Firesheep.
Anyway, what FireShepherd does is send out packets designed to crash all the running Firesheep in the network. It does this by sending out a malformed cookie. Essentially is is just exploiting bad programing in Firesheep and the Firesheep developer can easily update it to make it FireShepherd-prof.
If you really want to secure yourself though, FireShepherd is not the answer. You should consider using HTTPS.