The next time you get spare change, take a closer look at the coins. Who knows, you may get very lucky and land yourself an 1873-CC Liberty Seated Dime (valued at $2.5 million) or the 1913 Liberty Nickel (valued at $6 million):
Though it seems only a fool would pay upwards of six million dollars for a five-cent coin, this particular piece of metal is highly unusual because it was somehow produced without the knowledge of its maker–the U.S. Mint. Not much is known about the actual minting of the 1913 Liberty Head Nickels. Since the mints were to start production of the famous Buffalo Nickels that year, no Liberty Head type nickels were to be minted that year. However, the die for the nickel bearing the year 1913 had already been produced and delivered and it is believed that five specimens were struck at the Philadelphia mint before the die could be destroyed.
One theory says that the coins were struck as advance test pieces while another theory proposes that someone illegally struck the five specimens before the dies were destroyed for fun. In either case, it’s clear that the coins left the Mint in some unauthorized fashion, and didn’t surface until 1920, after the statute of limitations for theft had safely run out. Apparently, U.S. Treasury officials have concluded that they were legally struck, making it possible to own one of the five known examples if you can afford the price tag.
TopTenz has the list of the 10 Most Valuable American Coins: Link