It used to be called a "slug". I guess it's worth a penny. The quarter-sized ones are worthless, unless you can palm them off on somebody. It's a poor man's counterfeiting.
Casey

Annie, your coin is not a slug, it is a penny blank that was mistakenly released to circulation without being struck. I have one I got in change years ago. I would hold on to it, it is probably worth a dollar or two. A bookstore would give you information on this and other error coins.

Christopher

If a magnet sticks to it, it might be a penny blank. The only way to verify it is if it came out of a roll or bag of new pennies directly from the mint.
New cent coinage has copper over a ferrous metal core, since copper went up in price.
Casey

FYI: US cents have been made from copper plated zinc, a non-ferrous metal, since 1982. Canada produces a copper plated steel cent, but not the US. So, a magnet will NOT stick to a US cent blank, which this most likely is. It is uncommon to find, but not a great rarity.

C'mon, seriously, a penny slug? Who would bother counterfieting a dollar, never mind a penny!

Christopher

A way back when (in the 1950s) I worked after school at service station (filing station). It was my job to close the station before I went home.

On ocassion, we'd collect a slug in the vending machine. These slugs would pass as nickels in some machines. Our machine had a magnet inside to detect ferous material, and it hung onto these slugs. The downside, it also jammed the coin slot, and I'd have to find the hidden key and un-jam the coin slot. The trouble was, the boss, on ocassion, moved the key, and then the jam would have to stay until he came in. It got to be a cloak-and-dagger game to try to find who was passing these slugs.

The source of these slugs were knockouts from electrical wiring boxes. Those slugs were almost identical in size to a nickel.