Creepy is a software application that, when you type in someone's twitter username, downloads and gathers every tweet they've ever posted, every photo they've ever uploaded, extracts the location data from them (apparently this is recorded when you take a shot on a cell-phone), and displays the results in map form as above. So if they post alot from their house, or place of employment, you can find out where they live or work.
I've actually downloaded the software and lets just say its a bit buggy, not recognizing obvious usernames and encountering all sorts of errors when trying to down-load pictures. All the X girlfriends who dumped me have nothing to worry about. Nevertheless one twitter user, chosen at random from politwitter (and I'll leave their name out of it), gave the result above. A rather useful tool, under some circumstances. Also rather sinister.