1943 Steel Penny
1943. World War II. The U.S. mint is faced with a problem. Copper that would normally go into minting of pennies was crucial to the front lines of the war effort. The mint experimented with a number of stand-in materials: alloys, plastics, even Bakelite, before finally settling on zinc-coated steel.
The bright new coin was short-lived - the steel penny was only minted for a year. After many complaints, the mint used brass (recovered from spent shell casings) augmented with pure copper to bring back the pre-war spec penny in 1944.
You don't have to be a coin collector to pick one out of a handful of change, so I think that many non-numismatists have one squirreled away in their desk drawer or installed in a favorite pair of penny loafers. Their rarity isn't what you'd think since there were at least a billion of them minted, so if you do find one there's an outside chance it may be worth something like a buck.
However, there is a variant of the 1943 penny that is one of the rarest U.S. coins ever. When making the switch from copper to steel blanks, there were some copper planchets left in the hopper feeding the pressing machine. About 40 genuine 1943 copper pennies were minted, with 12 known to exist today. One just changed hands a few years ago for $200,000 (that's 20 million regular pennies, by the way).
The steel penny was unpopular in its day as it was frequently mistaken for a dime or a nickel. They could also rust in a sweaty pocket (eww.). They're probably worth more today as a conversation piece. Did you know that a steel penny is the only U.S. coin ever minted with no copper in it? It's also the the only coin you can pick up with a magnet. This was problematic back in its day as vending machines used magnets to pick out slugs... less of a problem today since you can't buy anything for a penny anymore. Not even a steel one.