Current websites vulnerable by Firesheep: Amazon, Basecamp, bit.ly, Enom, FaceBook, FourSquare, Github, Google, Hacker News, Harvest, The New York Times, Pivotal Tracker, Twitter, ToorCon, Evernote, Dropbox, Windows Live, Cisco, Slicehost, Gowalla, Flickr
Sites potentially soon to be supported by Firesheep: Yahoo, eBay, Linkedin, Digg, Reddit, Wikipedia, Blogger, GoDaddy, Posterous, Tumbr, Netflix, YouTube, SlashDot, MobileMe, PayPal, Salesforce, Craigslist, MySpace, Match, AOL
For those who don't want to install a Firefox plugin, you can simply make sure you use the https version instead of the http version in most cases. For instance,
https://www.google.com/* instead of
http://www.google.com/-j