Access is an installation that allows users to track individuals in a public space via a web interface.
"ACCESS is a public art installation that applies web, computer, sound and lighting technologies in which web users track individuals in public spaces with a unique robotic spotlight and acoustic beam system. The robotic spotlight automatically follows the tracked individuals while the acoustic beam projects audio that only they can hear. The tracked individuals do not know who is tracking them or why they are being tracked, nor are they aware of being the only persons among the public hearing the sound. The web users do not know that their actions trigger sound towards the target. In effect, both the tracker and the tracked are in a paradoxical communication loop. The ACCESS spotlight system travels from one undisclosed public space to another. The exact location of the public space is revealed only after ACCESS moves to its next location. The ACCESS website, which contains the webcam view and spotlight control, keeps an updated list of the locations visited as well as a video archive."
There are a number of videos available on their website, along with some still images of the installation in action and the web interface.
This is a reasonably long term project which has been installed in various locations since 2002.
I like the idea that the person being tracked doesn't know why they are being followed by the spotlight - paranoia kicks in!