Via Timo's excellent del.icio.us, Proximity Lab is an interesting participatory installation and experimental interface platform. It's meant to visualize relationships between users and mediated spaces.

Built on the premise that physical proximity is a basic unit of social communication, this study seeks to examine the role of physical interaction in social communication. (...) The purpose of this study is to examine the implications of physical proximity in social communication. The study seeks to stimulate inquiry on this topic through facilitated experiences where algorithmic logic, system observation of user behavior, and dynamic role assignment are central elements made accessible to participants for contemplation and discussion.

User perception and response to the overlay of information drawn from the fusion of user actions and system rules is also central to this study. (...) The platform is an 8-foot by 16-foot walkable surface fitted with radio frequency ID (RFID) technology. Participants wear shoes fitted with RFID tags, enabling the system to track and record their positions in real-time. Images projected directly onto the floor are accompanied by stereo sound as a continuous response to the actions and interactions of participants.

The website describes the test platform and shows nice visualizations:

If the changing positions of participants are the primary input for the system, then the visual material projected onto the platform floor is the primary means of output utilized by the system.