This site contains a combination of interactive simulations and lecture notes on harmonic oscillators, resonance, nonlinear dynamics, and chaos.  The simulations were designed to be adaptable to both algebra-based and calculus-based physics students, while the lectures target the calculus-based classroom.  Users can change variables such as the length of the pendulum, damping, amplitude, and frequency and view the result of the changes.  Help for users is available with a single click.  The lecture page offers four detailed chapters that explain the physics and calculus-based mathematics behind the pendulum simulations.