I love it when people do all of the work for you.
Firesheep is a FireFox plugin to hijack FB session, it looks really good.
There is a slide show here
With a short demo.
Here is a shot of the capture running.
This will work with twitter, facebook, and google.
Anything that does not keep an SSL connection is in trouble. So far it will support Windows and OSX but you need the PCAP libs installed for it to work.
Remember on Windows thats LibPCAP.