Firesheep is an add on for the Firefox web browser that will when using an unencrypted WiFi hotspot reveals information about other users sessions. I haven’t tried this myself but will add this add on when I next update my laptop. Given the growing spread of wifi hotspots this raises a range of security and privacy issues. I first heard about this on the Security Now Podcast. You can also learn more at Steve Gibson’s blog.
I strongly recommend listening to Security Now – when Steve Gibson is in full flight on technical detail I find I have to listen twice or rewind bits – but your persistence will be rewarded.