Firesheep

http://codebutler.github.com/firesheep/

What
A Firefox extension that demonstrates the insecurity of some web sites (including Facebook), allowing you to log on as another user on your local network.

How it works
Once you log in, these sites send you cookies to identify yourself. These cookies are sent unencrypted and anyone who intercepts them can pose as you.

I haven't fully tested it, but it seems to work. I could log into someone else's Facebook account. My network is 64-bit WEP.