Robot Simulators
Robot simulators are useful for rapidly test algorithms, design robots, and train AI system using realistic scenarios, etc.
Gazebo offers the ability to accurately and efficiently simulate populations of robots in complex indoor and outdoor environments. At your fingertips is a robust physics engine, high-quality graphics, and convenient programmatic and graphical interfaces. Best of all, Gazebo is free with a vibrant community.
Bullet is another popular robot simulator.