Newton's Aquarium 

Newton's Aquarium is an interactive physics package that helps students visualize and understand Newton's Law of Universal Gravitation. While creating and exploring their own solar systems, students encounter the core concepts in classical physics.
acceleration force gravity kinematics mass moon newton orbit physics planet speed star velocity weight

Gravit 

Gravit is a gravity simulator which runs under Linux, Windows and Mac OS X. It uses Newtonian physics. Although the main goal of Gravit is to be as accurate as possible, it also creates beautiful looking gravity patterns.
gravit graviton gravity nbody simulate simulation simulator octree octtree opengl sdl gl_arb_point_parameters