SPHERES are spherical satellites that are used inside the international space station to test instructions for spacecraft performing autonomous rendezvous and docking maneuvers. In addition, the SPHERES robots are used by high school students in the annual Zero Robotics tournament held by MIT and sponsored by NASA and ESA.
Source: MIT Space Systems Lab
SPHERES robot satellites on International Space Station (ISS) overview. An explanation of what the SPHERES are used for with some nice footage on board the ISS