Fellow student Luke Loeffler and I both presented Richard Serra and Nancy Holt’s 1974 "Boomerang" audio/video piece to separate classes at CMU. A delay line is used to throw Holt’s voice back at just the right speed to where her brain becomes confused and her speech and comprehension slows. She is filmed on live television as she explains the experience.
We made a simple Pure Data patch which replicates the experience. Plug in a microphone and headphones (preferably closed ear types) and give it a shot:
EDIT: The original patch was using a 100 ms delay. Later on I was told the original piece used 283 ms, calculated form research into cognitive perception, etc. I have updated the speed and, indeed, the effect is much more pronounced.
I took the Pd patch and then made an RjDj scene which can be run on an iPhone or iPod Touch. RjDj is a "reactive music" environment that runs live, interactive songs.
Install RjDj on the App Store and download Boomerang from the Soundtrips section. The effect works really well with earbuds, just make sure to turn up the volume. You can also make recordings and upload them to RjDj if you create an account.
EDIT: As of fall 2012, the RjDj app has been discontinued and is no longer available on the iOS App Store.
EDIT 2: A Boomerang for Pd scene is now included with PdParty for iOS.
Here is a transcription I made of the original video as source material: