Back in the good old days of the Cold War, spies didn't have encrypted mobile phones or digital thingamajigs to do their thing, so they did their spy business with classic spy stuff like spy camera-pens, spy shoe transmitters, spy bacon strips, and messages encoded on their spy underpants. Or these Hollow Spy Coins, which were used by the CIA and the KGB to hide poison or microfilms. Now you can buy them to store whatever is small enough to fit in them, like discarded nail bits. The coins are still in use by modern spies, however: Last year, the US Department of Defence cautioned its American contractors about hollow Canadian coins containing radio transmitters.
The Canadian coins were found by US defence contractors working on secret projects on three occasions between October 2005 and January 2006. Apparently, these were used to track movements of people carrying them. According to the experts, the coin transmitters could have been planted by China, Russia, or even France, not the Canadians, who actually said they didn't have a clue these existed.
These ones are harmless, however, and come in a variety of US and Soviet denominations, and you can even buy a Hollow Steel Spy Bolt and a Dead Drop Spike.
The hollow Dead Drop Spike was stamped into the ground by one spy, so it could be picked by another later. This one is made from a solid block of aluminium 3/4 inches in diameter.