I've got a lot of good mileage out of this one the last few years. Sent it to my dad at work 3 years ago, and he just called me and asked "What the F is wrong with you?" and hung up.
The best reactions are when you see the guy panicking trying to figure out how to turn it down. It used to be on youtube, but I can't seem to find it anymore. It definitely needs an more modern pop culture reference as an update.
http://www.toxicjunction.com/get.asp?i=V2849