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A robotic simulator can be as simple as a few matlab equations, or as elaborate as a photo-realistic 3D world with a physics engine. The type you use largely depends on your needs.

Wikipedia does a pretty good job describing the features of a few of the main simulators.

But here are some others:

1. Player Project (2D simulator - Stage - 3D simulator - Gazebo - and control interface - open source, part of the ROS project)
2. MORSE (general purpose indoor/outdoor 3D simulator)
3. Microsoft Robotics Studio (3D simulator with physics engine + control interface)
4. KiKS (Matlab plugin, only for Khepera + control interface)
5. MobotSim (for point like robots, more of algorithm implementation )
6. Karel (Pascal/Logo like)
7. Peekabot (More of a visualization tool)
8. MRPT (Mobile Robot Programming Toolkit)
9. Carmen (Robot Navigation Toolkit)
10. Webots (closed source, 3D simulation)
11. Simbad (2D/3D simulator in Java and Jython)
12. Robocode (A Java/ .NET suit)
13. Rossum's Playhouse (C/C++ suit)
14. V-REP (3D, source available, Lua scripting, APIs for C/C++, Python, Java, Matlab, URBI, 2 physics engines, full kinematic solver, etc.)
15. Simspark (RoboCup Soccer Simulation)
16. OpenRAVE (3D simulator, kinematics engine, planners, etc)
17. ODE (Physics simulator, many other simulators use this library)
18. MORSE (Modular OpenRobots Simulation Engine)
19. OpenHRP: Open source, tons of libraries written in C++ included for forward dynamics simulations and visualizations as well. Its a bit of a pain to get up and running though, quite a few dependencies that are very sensitive to the versions.
20. Robotran: Free for personal use and/or research uses. Operates primarily in the Matlab/Simulink environment. Very easy to get models up and running.
21. MapleSim: Commercial software but useful for very sophisticated modeling. Exports efficient symbolic multibody dynamics for simulation/visualization in Matlab/Simulink as well.
22. Breve (No longer maintained, 3D simulator for artificial life using Python)

Some more generic platforms/middlewares also offer simulation tools:

1. ROS
2. URBI
3. YARP
4. OROCOS (ORCOS (and its distant cousin Orca, orca-robotics.sourceforge.net) aren't simulators either. To the best of my knowledge ORCOS is a framework focused on real-time control, typically on a single computer, whereas Orca puts an emphasis on distributed components (with the obligatory own set of common robotics interfaces, of course). YARP (eris.liralab.it/yarp) is another framework similar in spirit to Orca, but focused more on vision. It's also not a robotics simulator, though.)

Some references on robot simulators:

1. Development environments for autonomous mobile robots: A survey
2. Open source robotics toolkits
3. Updated review of robotics software platforms
4. Existing Simulators - [email protected]
5. List at Asaf Matan's website

NOTE: The majority of this wiki page pulled from this question: http://stackoverflow.com/questions/2533321/robotics-simulator