General Resource Site:
Sound Hyperphysics

Musical Instruments
The acoustics of a flute (from the main page http://www.phys.unsw.edu.au/music/)

Java Sites to Check out:
Adding sine waves   Adding many sine waves (Fourier Synthesis)
Create Harmonics (overtones) and listen to them them!
More Harmonics
Even more Harmonics
Still More Harmonics
Beats (tuning your instrument)
Doppler Effect demo
Resonance (between "coupled oscillators")
Resonance Applet (2)Soundscape Applet - absolutely unique!
Sound Creation Applet - interesting?
Bell Ringing - great appeal?
Great Hyperstack of everything you ever wanted to know about sound!
Bagpipes!

Superposition of a Wave
Wave Interference
Wave Reflection

Tacoma Narrows Bridge Collapse
Video #1 (also has cool aircraft "flutter" videos)
A good basic report of the collapse
Another report of the collapse (Carlton U)
A pdf document on the collapse

Other (More Complex) Sites - just for "phun"?!
3D Wave interference (rotating box)
Fourier's Theorem - any shape wave can be produced from sine waves.
Virtual Physics Lab