I've seen little message board signature thingies that display the OS and (proxy) IP addresses of whoever's browsing that webpage. I figured that it was some sort of javascript function or other, but then recently I was talking to a script kiddie over aim, and he correctly told me my OS while rattling on about how 1337 he was. I wasn't directly connected with him, and I never gave him my IP address either.

So, my question is how can anyone figure out my OS like that? Any replies would be appreciated.