I think the word ‘intelligent’ is pushing it, but Britain’s Royal Mail service in conjunction with the Junaio app (iPhone or Android) has put out some stamps that can be used to trigger additional augmented reality-type info, this case in the form of videos.
Take your phone, open up the app and capture a photo of the stamp to launch the video (or other content). Here’s a video of it in action (skip ahead to 0:48 to skip the intro stuff).