The Crystal Robot is a two-dimensional self-reconfigurable modular robot system composed of Atoms. The Atoms are square and actuate by expanding and contracting by a factor of two in each dimension.
Two Atom versions have been produced in the Dartmouth Robotics Lab. Version 1 (first prototype) contracted/expanded both axes simultaneously. Version 2 (second prototype) has independent axis control as well as improved actuator motors, a more powerful microprocessor, and IR communication between adjacent Atoms.
The Crystal is also powerful in that all Atoms are identical, so reconfigurations as well as locomotion can be performed quite efficiently, as shown in these simulations: