Online photo management application Snapmania's Tourist Remover gets rid of unwanted gawkers and traffic from your photos by compositing two or more pictures of the same scene. All you have to do is make sure to take more than one of the same picture of a scene, assuming the people or cars you want to get rid of are moving. Once you've taken enough pictures so that every piece of the scenery is visible, just upload them to Snapmania and let the Tourist Remover do the work. Keep in mind that the more steady your hand when taking multiple shots, the better results you'll see, so ideally you'll be using a tripod or at least a wall to steady yourself. Alternatively, if you've got some GIMP or Photoshop chops, you can do the same thing yourself.
Tourist Remover [Snapmania via Photojojo]