Tom and Matt wrote about the remarkable phenomenon of ‘sine-wave speech’ in the Mind Hacks book (Hack #49) but I was just reminded of it recently (thanks Alex!) and I am always struck about what a great effect it is.
If you’re not familiar with it, I recommend psychologist Matt Davis’ webpage that explains the effect and has some fantastic examples.
Essentially, what initially sounds like random whistling sounds comes together as coherent speech when you know what you’re listening out for.
It’s a striking effect and is a wonderful demonstration of how prior knowledge and expectations can affect perception.
Link to Matt Davis’ sine-wave speech page.
Mind Hacks 
There are five examples there. Without any training I heard many of correct words of the first “random whistling sounds”. The last four did not come to me right away and do seem to be “random whistling sounds”. This could be the equivalent to Rorschach test.
A real-life example of this phenomenon in action is the strange case of the Satanic Muslim doll.
Absolutely remarkable.
My link disappeared. Oh.
http://www.foxnews.com/story/0,2933,435164,00.html
The URL seems to have changed: http://www.mrc-cbu.cam.ac.uk/people/matt.davis/sine-wave-speech/