Instructions for installing ROS on the Raspberry Pi are available for several distributions:
Installing ROS Fuerte: ROSberryPi/Installing ROS Fuerte on RaspberryPi
Installing ROS Groovy: groovy/Installation/Raspbian
Installing ROS Hydro: ROSberryPi/Setting up Hydro on RaspberryPi
Installing ROS Indigo: ROSberryPi/Installing ROS Indigo on Raspberry Pi
Installing ROS Kinetic: ROSberryPi/Installing ROS Kinetic on the Raspberry Pi
Installing ROS Melodic: ROSberryPi/Installing ROS Melodic on the Raspberry Pi