peekabot is a versatile 3D visualization tool targeted at robotics researchers and developers. It's multi-platform, general purpose, easily extendable, scalable, and does not depend on any other frameworks to function. It also provides some functionality for interacting with your robots using the GUI. You can test your algorithms in both simulated and real environments. The only requirement is that the device can use TCP/IP.
ARS (Autonomous Robot Simulator) is a physically-accurate simulation suite for research and development of mobile manipulators and, in general, any multi-body system. It is modular, easy to learn and use, and can be a valuable tool in the process of robot design, in the development of control and reasoning algorithms, and in teaching and educational activities. It will encompass a wide range of tools spanning from kinematics and dynamics simulation to robot interfacing and control.