Windows/Linux: If you've ever doubted how much anyone could track you based on information embedded in geolocation services like Foursquare, Twitter and Flickr, free app Creepy will show you by aggregating GPS coordinates for any user, pointing out their most frequented hangouts on a map.
Let's say you wanted to know where a specific Twitter, Foursquare or Flickr user spends the majority of her time; Creepy lives up to its name by showing you. Just type in any username for a supported service and it will grab any and all location data it can find, from Foursquare check-ins, tweet locations, and even the EXIF data from photos posted to Flickr, Twitpic, yFrog and other image services. Essentially, it's a stalker's dream app.
Like previously mentioned Firesheep extension for Firefox, consider this app as more as an interesting proof-of-concept (we trust you to use it for good!). Perhaps you didn't realise how accurately your tweets can pinpoint your house or favourite hangouts, or perhaps you didn't even know you were sharing location data in all your uploaded photos (easy to remove if you know how). Punch in your own accounts and see how much shows up—you might be surprised.
Creepy is a free download for Windows and Linux.