I'd like to introduce you a robot simulator, that I have been developing in the last few years:
V-REP - the Virtual Robot Experimentation Platform - is a 3D robot simulator, with integrated development environment, based on a distributed control architecture: control programs (or scripts) can be directly attached to scene objects and run simultaneously in a threaded or non-threaded fashion. This makes V-REP very versatile and ideal for multi-robot applications, and allows users to model robotic systems in a similar fashion as in reality - where control is most of the time also distributed.
V-REP allows you to edit and simulate whole robotic systems, or sub-systems (e.g. sensors, mechanisms, etc.). It offers a multitude of functionalities (e.g. 2 physics engines, full forward/inverse kinematics solver, path planning, mesh-mesh minimum distance calculation module, proximity sensors, camera sensors with image processing, distributed control, etc.) that can be easily integrated and combined through an exhaustive API and script functionality.
V-REP is used for remote monitoring, hardware control, fast prototyping and verification, fast algorithm development / parameter adjustment, safety double-checking, robotics-related education, factory automation simulations, etc.
V-REP is free for students and can be downloaded here: http://www.v-rep.eu
I posted a demo video on the "Robot videos" forum page, that gives a good idea of V-REP's functionality.