Brusspup filmed water going through a speaker running at 24Hz (Hertz) [1] (link via Hacker News [2]) and the result is incredible—the water just sits there, in mid-air. As he adjusts the sound wave, the water appears to move slowly down, then reverses itself.
It is a trick, though it's not special effects—it's all done in real time with a camera. But it's the camera that causes the effect—it's acting like a strobe light, catching the action at just the right moment. And it's still impressive though.
[1] http://www.youtube.com/watch?v=uENITui5_jU