Robot Operating System is robotics middleware (i.e. collection of software frameworks for robot software development).
Although ROS is not an operating system, it provides services designed for heterogeneous computer cluster such as hardware abstraction, low-level device control, implementation of commonly used functionality, message-passing between processes, and package management.
Free to download and well supported (with a large international robotics community in many industries), and software connectors are usually available in simulators and test packages.
ROS was first used in an autonomous car in 2010.