Beats: the superposition of waves of slightly different frequency.
Standing waves: the superposition of two waves travelling in opposite directions.
Doppler effect: moving source.
Interference of waves from point sources: use two sources, which you can drag around to different separations, or add more!
Electric field applets:
Schaum Outline: Physics for Engineering and Science
Action potentials turned into pulses of sound
Maxwell in the Economist.
Wikipedia on polarization