These links lead to collections of physics learning tools, mostly relating to waves and oscillations. Look for expansions to this post in the future.
A set of neat little applets from Walter Fendt. Scroll down to the oscillations and waves section to see simulations of resonance, coupled pendula, beat frequencies, etc.
A set of physics applets from the University of Colorado, including waves, motion, quantum, light, and many others.
A “friction harp” that uses resonance to produce tones. “Tis a Gift to be Simple”
A really nice simple harmonic motion demonstration at CoLoS, Conceptual Learning of Science, showing a revolving circle linked to an oscillating spring. Very clear depiction of displacement, velocity, and acceleration.
Physics demonstration videos from Wake Forest University Dept. of Physics.