Small robots called SwarmBots

SwarmBots communicate with their neighbors using a set-based neighbor query system and a gradient-based ad-hoc messaging system. Global behaviors are formed from the interactions of many individuals by using iRobot’s collection of "group behavior building blocks." iRobot developed a large library of these behaviors, and has used them to perform group tasks such as clustering to a location of interest, surrounding an object to measure its perimeter, navigating long distances using neighboring robots as landmarks, and exploring and mapping a large building.

http://www.irobot.com/sp.cfm?pageid=149