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...
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
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.
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.