The Spy Coin($36) is a hollow coin that you can transfer messages or place a tiny transmitters. "Oh Boy: Jan. 11, 2007: The U.S. issues a spy coin warning after discovering tiny transmitters planted in Canadian coins. The government said the mysterious coins were found planted on U.S. contractors with classified security clearances on at least three separate occasions between October, 2005 and January, 2006 as the contractors traveled through Canada."
