Synthtopia posts about a cool speaker feedback array. Each of the five differently sized speakers are paired with a piezoelectric contact mike. An on/off switch and volume knob control each pair.
Anyhow, Synthtopia explains it well, talks about the gizmo's creator and has two performance videos, which I like playing (more or less) simultaneously.
Speaking of more or less simultaneously, Synthtopia also posted this video of one pianist playing Steve Reich's Piano Phase on two pianos.
And along similar lines of synchonization & feedback loops, a vid about porn Foley artists, which is either entirely for laughs or these Foley guys dabble in comedy as well.