The N-Body-problem by D. Rapaport

A nice 2d applet which also lets you study nonstable motions depending on the starting parameters of N planets. Adjust the planets initial velocity and position in a plane and see what happens...

Kepler applet by David McNamara and Gianfranco Vidali

Another fine 2d applet which emphasizes on kepler's second law. The coordinate system is attached to the sun. Realtime visualization of velocity and acceleration vectors

Kepler's Laws from Roberto De Leo

A 2d applet which lets you adjust the start position and velocity of a planet. The coordinate system is attached to the sun. Includes source code.

Kepler's Third Law by David Caley

A 2d applet visualizing kepler's third law (e.g. the relation of the ellipse "radius" (semimajor axis) to the orbital period). Shows two planets on adjustable circular paths around the sun.