Any new computer system organization raises many questions about hardware, system software, algorithms, and programming; multicomputers are no exception. The commercial emergence of hypercubes has made it possible to move many research questions from theoretical to experimental venues. However, hardware availability does more than permit implementation of previously tested ideas. The feasibility of many application and operating system algorithms can only be determined experimentally. We believe it is difficult, if not impossible, to experimentally examine these issues singly - they interact in both obvious and subtle ways. This paper describes the Picasso hypercube operating system, a part of the Picasso project at the University of Illinois. Motivated by the need for a flexible operating system testbed, the design of Picasso has exposed several important, and necessary, features of multicomputer operating systems designed for research. We present the design and current performance of Picasso, discuss the lessons learned, and conclude with an overview of future research plans.