Among other things, Firesheep can easily intercept other users’ login details for Facebook and Twitter, among other things.
From the security company Zscaler there is now a plugin called Blacksheep. Blacksheep is a small plug-in for Firefox that can detect if someone on the local wireless network is trying to eavesdrop on your computer using Firesheep.
However, Blacksheep is not a waterproof protection against Firesheep and can only give you a warning, but not protect against your data being intercepted. The safest solution so far is to use a so-called vpn connection or to completely avoid unencrypted wireless networks.