BlackSheep add-on detects Firesheep on your network
If someone is using Firesheep, BlackSheep will trick it with a fake login cookie. When Firesheep takes BlackSheep's bait and tries to get your user information from a site using the fake values BlackSheep has been sending out, you'll get a warning that Firesheep is operating, as well as the IP address of the person using it.
Unfortunately, BlackSheep is only available for Firefox at the moment. There are other security measures you can use, though. Sebastian has posted a roundup of ways to surf securely with SSL in any browser. Even more recently, someone released a Safari extension to force Facebook to use SSL. As far as direct anti-Firesheep countermeasures go, a Windows app called Fireshepherd can also help you shut Firesheep snoopers down.