The "Reset" button brings the body of pendulum to its initial position. You can start or stop and continue the simulation with the other two buttons. If you choose the option "Slow Motion", the movement will be five times slower. The spring constant, the mass and the amplitude of the oscillation can be changed within certain limits. In order to select another physical size you have to click on the appropriate one of the five radio buttons. Source code available.
Foreign language versions available.