(From Wikipaedeia) A Geocoin is a token used in geocaching. They can be used for one of several purposes.
They can be a signature item used by geocachers, left in geocaches to be found by others and kept as trophies or traded. Geocoins are often minted with the name of their state or country of origin, or with a given geocacher's name or screen name. They can also be imprinted with unique codes and used in a similar way as Travel Bugs, where they can be left in geocaches to be moved to other geocaches and then tracked via a geocaching website. Many coins also have a unique icon shown on the geocaching website beside a cache the coin is in. Some geocoins may stay with their owners and travel with them from cache to cache. Due to the fact that many geocoins have unique icons, many people bring such geocoins to geocaching events so that others may see the coins and then collect the icons as part of their stats.
In recent years, the geocoin craze has taken off dramatically, and there are hundreds, if not thousands of different geocoins out there today. The geocoin section of the Groundspeak Forums  is one of the most active on the site.
Also, many geocoins are sold on eBay, and a simple search there will provide a collector with many rare and sold out geocoins and occasionally a great deal.