The Modlab at Penn designed the CKBot (Connector Kinetic roBot) module to be fast, small, and inexpensive. These qualities enable it to be used to explore the promise of modular robotics systems, including adaptability, reconfigurability, and fault tolerance. They've researched dynamic rolling gaits, which use a loop configuration to achieve speeds of up to 1.6/ms, as well as bouncing gaits by attaching passive legs. They are also using the CKBots to research the difficult problem of configuration recognition, and, for the Terminator 2 fans, they have even demonstrated "Self re-Assembly after Explosion" (SAE).
More recently, Modlab has developed ROS packages that can be used when the CKBots are connected to a separate ROS system. They have also created an open source repository, modlab-ros-pkg, for CKBot ROS users. The CKBot modules only have a few PIC processors -- not enough to run ROS -- so an off-board system enables them to use algorithms that require more processing power. In one experiment, they used a camera to locate AR tags on the CKBot modules. The locations were stored in tf, which was used to calculate coordinate transforms between modules. They have also used rviz to display the estimated position of modules during SAE when AR tags were not in use.
One of the projects Modlab is currently working on is a "mini-PR2" made out of CKBot modules. The mini-PR2 will be kinematically similar to the Willow Garage PR2 and is powered by a separate laptop. You can see an early prototype of mini-PR2 opening an Odwalla fridge:
CKbots trace their ancestry back to Professor Mark Yim's work on the PolyBot system at PARC. The PolyBot system had an impressive range of demonstrations, including fence and stair climbing, tricycle riding, and even transforming from a loop, to a snake, to a spider.