Waves
Funny Voices - What happens when you breathe in gas which is denser than air?
Brainiac Orchestra - Wind instruments played with Helium
Slow motion waves video - showing waves on a guitar string etc.
Pinhole camera - how to make one in your room
Invisible Man - Trailer from the original movie
Sofar - A very long (and occasionally boring) MIT lecture, the last bit is the best
?Volcano Eruption - by Phil MacNamara