Cyberbotics recently announced that Webots, a cool robot simulation software that has been developed for more than 20 years, is now opensource and available for free download. Now everyone can create their own personalized robot.
Accurate physics simulation allows creating walking, wheeled, flying or swimming robots as well as modular systems which can be (de)coupled using mechanical connectors. You can set up robot parameters manually or derive them using evolutionary algorithms (not sure whether the last video uses Webots but looks very similar).
Documentation and tutorials, libraries of ready-made robots, sensors, actuators and static objects are available for your disposal. Webots supports different operating systems and integrates with many programming languages.
Once when I was a student I was playing with it, fitting a passive dynamic walker for a specific slope.
So, enjoy! Here is the link.