We present Medusa, a proximity aware multi-touch tabletop. Medusa uses 138 inexpensive proximity sensors to: detect a user’s presence and location, determine body and arm locations, distinguish between the right and left arms, and map touch point to specific users and specific hands. Our tracking algorithms and hardware designs are described. Exploring this unique design, we develop and report on a collection of interactions enabled by Medusa in support of multi-user collaborative design, specifically within the context of Proxi-Sketch, a multi-user UI prototyping tool. We discuss design issues, system implementation, limitations, and generalizable concepts throughout the paper.