In this paper, we explore the interaction space of MobiLimb, a small 5-DOF serial robotic manipulator attached to a mobile device. It (1) overcomes some limitations of mobile devices (static, passive, motionless); (2) preserves their form factor and I/O capabilities; (3) can be easily attached to or removed from the device; (4) offers additional I/O capabilities such as physical deformation and (5) can support various modular elements such as sensors, lights or shells. We illustrate its potential through three classes of applications: As a tool, MobiLimb offers tangible affordances and an expressive controller that can be manipulated to control virtual and physical objects. As a partner, it reacts expressively to users' actions to foster curiosity and engagement or assist users. As a medium, it provides rich haptic feedback such as strokes, pat and other tactile stimuli on the hand or the wrist to convey emotions during mediated multimodal communications.