As performed by Doug Henning on "The Tonight Show" with Johnny Carson!  This is not a secret to be share with friends at a party.  This is one to be closely guarded.  The effect on the audience is really hard to believe.  

Basic Effect: You display a small envelope with a prediction inside.  A volunteer is handed a deck of cards to deal onto the table turning each card face up on the stack.  They can stop at any time.  When they do stop you open the envelope.  There is a playing card inside... a duplicate of the card they stopped on.

Perfect it, perform it, protect it!  It will treat you well!