AstroGrav (Mac)
AstroGrav is a software program that allows you to study how astronomical objects move and interact under the force of gravity. It features superb interactive 3D graphics which allow you to move around within an astronomical system that you are viewing, while watching how it evolves with the... AstroGrav calculates how astronomical objects move according to Newton's Law of Universal Gravitation, and displays the results as they are calculated so that you can watch the system evolve as in a movie.
http://www.astrograv.co.uk/AstroGrav.dmg - 1Mb - Download - Screenshot

AstroGrav (Windows)
AstroGrav is a software program that allows you to study how astronomical objects move and interact under the force of gravity. It features superb interactive 3D graphics which allow you to move around within an astronomical system that you are viewing, while watching how it evolves with the... AstroGrav calculates how astronomical objects move according to Newton's Law of Universal Gravitation, and displays the results as they are calculated so that you can watch the system evolve as in a movie.
http://www.astrograv.co.uk/AstroGrav.zip - 1Mb - Download - Screenshot