there are a lot of Fake commercials and spoofs out there. These ones are the ones I consider the top 5. (obiviously I havent seen all of them so I probably missed some)
Edit. Due to some Comments on Digg.com I realize Cat herders should not be here as it is real. To Make up for it I am adding one that I completely forgot about and is truly one of my favorites:
Schmidts Gay from SNL
Schmidt's Gay SNL
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Carlsberg and Mentos
And the number one POP ROCKS!