You can enter a Twitter or Flickr username into the software's interface, or use the in-built search utility to find users of interest. When you hit the 'Geolocate Target' button, Creepy goes off and uses the services' APIs to download every photo or tweet they've ever published, analysing each for that critical piece of information: the user's location at the time.
While Twitter's geolocation setting is optional, images shared on the service via sites like Twitpic and Yfrog are often taken on a smartphone - which, unbeknownst to the user, records the location information in the EXIF data of the image. Creepy finds these photos, downloads them, and extracts the location data.
When the software finishes its run, it presents you with a map visualising every location that it found - and that's when the hairs on the back of your neck go up. While the location of an individual tweet might not reveal much, visualising a user's history on a map reveals clusters around their home, their workplace, and the areas they hang out. Everything a stalker could need, in other words.
Unfortunately, it's not yet available for Mac, so if one of my readers would like to try it out (perhaps using your own Facebook/Flickr/other details rather than another person's, I'd love to hear how effective (and creepy) it really is.