Harmonic Motion

Coupled Oscillations
This applet illustrates coupled oscillations of a linear chain of identical noninteracting bodies connected to each other and to fixed endpoints by identical ideal springs. All bodies start from rest, and their initial positions can be set either by sliding them along the track
http://lectureonline.cl.msu.edu/~mmp/applist/coupled/osc2.htm

Damped Harmonic Motion
Mathematical Equations of Damped Harmonic Motion.
http://musr.physics.ubc.ca/~jess/hr/skept/SHM/node7.html

Simple Harmonic Motion
A series of graphical animations illustrating the features of this phenomenon, with online consolidation exercises.
http://www.physics.uoguelph.ca/tutorials/shm/Q.shm.html

The Simple Plane Pendulum
This applet illustrates the simple plane pendulum, with or without damping. The user enters the damping coefficient and initial conditions, and the applet animates the pendulum's motion and plots the angular velocity versus the angle. Such a plot is called a phase portrait.
http://www.iopb.res.in/~vshenoi/scienceday/fundemoapplets/JAVA/pendulum/pendulum2.html